Personality and relevant background from amatomnes app
If one were to describe him in a word, it would be conscientious: he understands all too well the gravity of his duty to protect his country, and is willing to sacrifice everything of himself and endure any humiliation to carry it out. He differs from almost all of the novel's other generals in that he truly doesn't care about anything other than the greater good of his country - though occasionally mocked for his cautiousness in defending against Zhuge Liang, he prefers to avoid battle where victory is uncertain, and because he knows Shu lacks resources and has frequent supply problems, he often resorts to simply waiting the enemy out. This attitude stems from the fact that, unlike many of his comrades, who had spent the better part of their lives waging war across the land, Sima Yi spent most of his career in Wei as a civil official, and was known for backing policies that would benefit the common people in addition to strengthening the government, such as the wide-scale implementation of the tuntian system and the establishment of schools throughout the northern heartland. Though this meant he was the only one who could defend effectively against Zhuge Liang, as he was the only one who wouldn't allow himself to be provoked into suffering defeat through rash action, it also meant that he was emotionally unprepared for the rigors of war. Some of the incidents that are most illustrative of his character occur in chapter 100 of the novel; however, the canon point I'm pulling him from is three years after this, so I'll also include an explanation of how his character develops after that.
Achieving glory in battle is not something that he attaches great importance to, so long as his country remains secure - he assigns credit to his comrades and puts them in positions where they can be the ones to achieve crucial victories whenever possible, and shows concern for his men and officers on a personal level rather than solely because of the asset to Wei's defenses they represent. For example, at the battle of Jieting (chapter 95), he takes most of the enemy's strongpoints, beating Cao Zhen and Guo Huai's forces to the punch; afterward, not only does he disavow that he was out for personal glory, but he makes up for depriving them of it by providing them with a campaign strategy that should allow them both to win crucial victories (and having the messenger who delivers it neglect to mention that it was Sima Yi's idea).
In chapter 100, when Cao Zhen is no longer sulking at him and the two of them take the field together again, Sima Yi anticipates that Zhuge Liang will attack a certain strongpoint; Cao Zhen doubts his judgment, so Sima Yi makes a bet with him to convince him to stay and defend the pass (with Sima Yi's offer being that, if Zhuge Liang hasn't attacked within ten days, Sima Yi will come to Cao Zhen's camp dressed in drag to apologize; Cao Zhen accepts, making this the first - but not the last - time one of the novel's other characters tries to get Sima Yi to crossdress - and, later in the chapter, the contest of battle arrays which he loses to Zhuge Liang was to "decide who is the rooster and who is the hen"). The ensuing series of events is highly illustrative of Sima Yi's character, and especially of his selflessness and devotion to the greater good - the latter of which remains his driving force throughout the novel.
He dresses as a rank-and-file soldier to inspect how secure the defenses really are, and along the way, he overhears an officer complaining about the generals making wagers amongst themselves when the campaign has not been a spectacularly successful one to begin with; Sima Yi has the officer executed for weakening morale, so as to make sure that he has his men's full attention when he explains to them that "I'm not 'laying wagers.' I'm trying to defeat Shu's army so that you can all go back to the court and claim your rightful reward for distinguished service." Thus, he succeeds in driving home the seriousness of the situation without, as might be expected in the wake of such an incident, further driving down morale; this is the first of the incidents in the chapter that demonstrates that, even when he's being a hardass, his foremost concern is always for the welfare of his country and its people.
His equal concern for his comrades is illustrated clearly not long thereafter. When Shu's troops arrive not long afterward and he manages to successfully ambush and defeat them, he immediately sends a messenger to Cao Zhen, telling him to forget about their bet and maintain the utmost vigilance; when Cao Zhen reports that he hasn't seen any hint of Shu's forces, Sima Yi correctly infers that they're planning to launch a surprise attack, and immediately comes to Cao Zhen's aid, though not in time to keep him from suffering a serious defeat. After rescuing Cao Zhen, he doesn't reproach him for his incaution, but instead tries to get him to focus on salvaging the Wei forces' position, adding, "Let's have no more talk about our wager, but serve our kingdom with united devotion." This incident again demonstrates how completely he's put aside his ego, as well as a great deal of patience and forbearance - the last of which he initially offers to Wei's enemies as well, for the sake of trying to give the country a chance to recover from what, at that point, has been fifty years of nearly unabated civil war.
Not long after his defeat, Cao Zhen falls ill and dies, and Sima Yi ends up doing battle with Zhuge Liang again; when they are exchanging challenges, Sima Yi doesn't boast about how he's going to wipe Shu and Wu off the face of the earth, but instead, offers peace if Zhuge Liang is willing to accept it: "But you and your commanders can still save yourselves if you will reflect and alter your course, withdraw now and let our three separate kingdoms defend their borders in a settled order, thus sparing the common souls of our land the miseries of war." Then, rather than threatening to catch Zhuge Liang and execute him, he demands that, if he loses, he retire and return to his village, "under my guarantee of safety, of course." Zhuge Liang then proceeds to beat his capacity for mercy out of him, starting with the ensuing battle; in the initial clash between their forces, Sima Yi's battle formations are bested, so he brings his reserves into the fight, personally leading the charge (demonstrating that, while he doesn't relish bloodshed, his distaste for it stems from compassion rather than cowardice, and he's capable of holding his own in battle if need be). Again, he comes off the worst, and thereafter, he adopts considerably more cautious tactics in dealing with Zhuge Liang. This caution is cemented after, when Zhuge Liang is withdrawing his forces from the field at the end of this campaign (the fourth of his northern expeditions), Sima Yi's vanguard general, Zhang He, convinces him to undertake a pursuit (the incident in the novel differs from the historical version of the incident Wikipedia lists); in the process, however, Zhang He is ambushed and killed, leaving Sima Yi with almost no one he could rely on in framing a strategy to defeat Shu.
With two of his trusted colleagues - both of whom were his close friends as well - deceased, and with Zhuge Liang having finally succeeded in getting inside his head, the fifth and final of Zhuge Liang's northern campaigns proved to be the one that nearly broke Sima Yi (though, he got off comparatively lightly, given that it killed Zhuge Liang). While the first victory of this particular campaign went to Sima Yi, Zhuge Liang soon more than made up for his losses, finally solved Shu's supply problems (thus rendering useless Sima Yi's fallback strategy of waiting the enemy out), and took advantage of Sima Yi's policy of sending captured enemy troops home to feed him small victories in order to overcome his completely justified fears that Zhuge Liang would find a way to entrap him if he did anything other than defend - and then, once Sima Yi was confident enough to stage another offensive, Zhuge Liang came within a hairsbreadth of killing him and his two eldest sons with one of his infamous fire attacks. Unable to attack, Sima Yi returned to a strictly defensive posture despite Zhuge Liang's repeated insults and provocations (which weren't easy to shrug off - by this point in the novel, Zhuge Liang had killed three guys just by insulting them; I am not making this up) and the growing unrest of his own troops; the novel repeatedly mentions his despair at the situation, until he figured out that Zhuge Liang's health was failing.
Even once Zhuge Liang finally died, Sima Yi worried that news of his death was only another ruse, and pretty much shat a brick when Shu's army pretended it was; though he tried to laugh it all off and pretend that everything was fine now that Wei was safe, later incidents in the novel demonstrate that Wuzhangyuan continued to prey on his mind well after the fact. Though he had won, the nature of his victory did little to restore his confidence, and destroyed any capacity for compassion he had toward those who would disturb the land's peace. For example, three years afterward (and after the canon point I'm pulling him from, though I feel this bears mention anyway) he pushes his troops to the limit in putting down Gongsun Yuan's rebellion in Liaodong, and rather than offer him any kind of mercy, he drags out negotiations until Yuan's army is out of supplies, and once the enemy has lost the will to fight, isolates Yuan and executes him along with his entire family and all of his adherents.
As I mentioned above, Sima Yi's primary loyalty is seen to be to the good of his country, rather than the interests of his ruler - only natural, as hard as he has worked to make the state of Wei great, and given that he takes quiet pride in his accomplishments for its sake. Because of this and his considerable talents as a strategist and statesman, his relationship with Emperor Cao Rui hasn't been the best, as the latter finds his ability intimidating. For his part, Sima Yi is underwhelmed by the young emperor, and knows his own skill at managing the state is far better - and on some level, he still feels slighted by him for believing Zhuge Liang's ploy to get Sima Yi dismissed from office on suspicions of treason. However, following Sima Yi's steadfast service in defending against Shu, and due to Sima Yi's tremendous patience, relations between them are improving - and even if they weren't, going into rebellion is something that Sima Yi would never even begin to contemplate (as shown by the fact that, when he's accused of it, though he more than has the means to rebel, he sends his troops back to their garrisons and goes alone to get the situation sorted out with Cao Rui, knowing it could well get him killed). As I said, this is due in part to how important his service to Wei has been to him; however, it also stems from the memory of his close friendship with Cao Rui's father and Wei's first emperor, Cao Pi, as well as his unreserved admiration of Cao Pi's father, the famous warlord Cao Cao.
When he first entered the government, Sima Yi soon became friends with Cao Pi - though Cao Pi didn't live up to his father's brilliance as a statesman (no one in the era ever did; even Zhuge Liang failed to provide for his country's continued prosperity and survival after his death), Cao Pi was shrewd and tenacious, and made up for any shortcomings in his personal abilities by humbly accepting the wisdom of his advisors; he knew that if he was to compare with his father's reputation, he had to make use of every advantage at his disposal. Sima Yi admired his dedication, considered him the best choice to run the country when Cao Cao was gone, and played a role in persuading Cao Cao that he should designate Cao Pi as his heir; later on, he advised Cao Pi in forcing the last Han emperor's abdication. In turn, Cao Pi always seems enthusiastic about Sima Yi's plans and ideas in the scenes they share together, and when Cao Pi takes the field to lead an invasion against Wu, Sima Yi is the one he leaves running the government, despite the fact that Jia Xu, one of his oldest and most trusted advisors, is still alive at that point and might seem like the more natural choice. However, Cao Pi's confidence in Sima Yi is absolute, and if Cao Pi actually liked any of his blood siblings, it could be said that he treats Sima Yi like a brother.
If Cao Pi is his adoptive brother, Cao Cao is like a second father to Sima Yi; their meeting was one that changed Sima Yi's life and his outlook forever, and he admired Cao Cao from the very beginning. Originally, Sima Yi never intended to serve in the government; he saw no reason to sacrifice himself for a failing dynasty, especially when there were a surprising number of officials he considered at least as competent in affairs of the state as himself already doing that. He would rather have left the government work to his elder brother and remained at home to watch over his family, and retain at least some measure of control over his own destiny that way; perhaps his uncommonly thorough knowledge of astrology had given him some insight into the kind of suffering that lay ahead for him after he chained his fate to the country's. However, his first meeting with Cao Cao made a powerful impression on him. His attitude was far from that which had characterized most of the Han officials before him; while his brilliance, determination and magnetic personality made him an almost irresistible force, his own personal interest was a distant secondary concern when it came to how he ran his government. His primary concern was for rebuilding the country, and returning peace and stability to the lives of its citizens - and he had practical, concrete plans for how to accomplish just that. In him, Sima Yi found someone he could admire without reservation, and it was from Cao Cao that he learned what it meant to be devoted to an ideal that was bigger than oneself. For his own sake, Sima Yi had no ambition, but for Cao Cao, he wanted to conquer the entire realm - and Cao Cao valued him highly not only as an advisor, but on a personal level as well. For example, there is one scene wherein Liu Bei has just sent a message to the emperor, proclaiming that Cao Cao is a rebel and Liu Bei intends to destroy him. Cao Cao immediately goes into a rage, and prepares to muster his troops against Liu Bei, when Sima Yi checks him, and indicates that he has a better way in mind to deal with the situation; in other, similar scenes in the novel, this kind of thing has only been shown to make Cao Cao angrier, but this time, it completely turns his mood around (his reaction is translated by Moss Roberts as "Cao Cao asked amiably, 'Zhongda, what is your esteemed view?'").
However, despite his personal affection for Sima Yi, Cao Cao realized that the two of them were greatly alike, and that could lead to problems down the line; while he knew that Sima Yi's loyalty to him and to Cao Pi was absolute, he also knew that Sima Yi wasn't the type to suffer fools gladly (for example, he refused overtures of friendship from Cao Hong, whom Sima Yi considered an idiot and whose character he held in contempt, despite the fact that Cao Hong was a close friend of Cao Cao's), valued the common good higher than the whims of his ruler, and would chafe under a lord whose competence and ability he could not respect. Moreover, he saw that once Sima Yi reached a certain level of influence, the growth of his power would be difficult to check, as Sima Yi's immense talent meant that he would inevitably outperform the scope of his responsibilities, and his knack for recognizing, promoting, and winning the friendship of men of ability (and when it comes to whom he associates with, ability is all that matters to him - for example, he's close friends with Cao Zhen, whom several of the other figures in the novel regularly tease for being overweight, and he mentors Deng Ai, who is socially awkward due to a speech impediment; as for his ability to win people over, Guo Huai initially dislikes him, probably due to Sima Yi's lack of military experience when he first takes the field, but by the time of Wuzhangyuan, the two of them have become friends) meant that he would naturally fill the administration with his adherents. Cao Cao knew firsthand the dangers of any one faction becoming too dominant in the court - after all, he was puppeteering Emperor Xian at the time. Not trusting Cao Pi's successor to be competent enough to control Sima Yi, and not trusting Sima Yi to always be a better man than him, Cao Cao took the same precautions in dealing with him that he would against himself. Sima Yi understood his worries well, and throughout the novel, is shown to be consistently striving not to follow in Cao Cao's footsteps.
After his duty to his country, family is the most important thing in the world to Sima Yi, and the Sima clan as a whole is extremely close-knit. This is most readily apparent in Sima Yi's relationship with his two eldest sons, Shi and Zhao. When Sima Yi becomes one of the novel's central figures, his sons are always by his side; he relies on their judgment on numerous occasions during the war with Shu, and throughout the rest of the novel, they continue to be the only ones he is shown to trust implicitly.
Achieving glory in battle is not something that he attaches great importance to, so long as his country remains secure - he assigns credit to his comrades and puts them in positions where they can be the ones to achieve crucial victories whenever possible, and shows concern for his men and officers on a personal level rather than solely because of the asset to Wei's defenses they represent. For example, at the battle of Jieting (chapter 95), he takes most of the enemy's strongpoints, beating Cao Zhen and Guo Huai's forces to the punch; afterward, not only does he disavow that he was out for personal glory, but he makes up for depriving them of it by providing them with a campaign strategy that should allow them both to win crucial victories (and having the messenger who delivers it neglect to mention that it was Sima Yi's idea).
In chapter 100, when Cao Zhen is no longer sulking at him and the two of them take the field together again, Sima Yi anticipates that Zhuge Liang will attack a certain strongpoint; Cao Zhen doubts his judgment, so Sima Yi makes a bet with him to convince him to stay and defend the pass (with Sima Yi's offer being that, if Zhuge Liang hasn't attacked within ten days, Sima Yi will come to Cao Zhen's camp dressed in drag to apologize; Cao Zhen accepts, making this the first - but not the last - time one of the novel's other characters tries to get Sima Yi to crossdress - and, later in the chapter, the contest of battle arrays which he loses to Zhuge Liang was to "decide who is the rooster and who is the hen"). The ensuing series of events is highly illustrative of Sima Yi's character, and especially of his selflessness and devotion to the greater good - the latter of which remains his driving force throughout the novel.
He dresses as a rank-and-file soldier to inspect how secure the defenses really are, and along the way, he overhears an officer complaining about the generals making wagers amongst themselves when the campaign has not been a spectacularly successful one to begin with; Sima Yi has the officer executed for weakening morale, so as to make sure that he has his men's full attention when he explains to them that "I'm not 'laying wagers.' I'm trying to defeat Shu's army so that you can all go back to the court and claim your rightful reward for distinguished service." Thus, he succeeds in driving home the seriousness of the situation without, as might be expected in the wake of such an incident, further driving down morale; this is the first of the incidents in the chapter that demonstrates that, even when he's being a hardass, his foremost concern is always for the welfare of his country and its people.
His equal concern for his comrades is illustrated clearly not long thereafter. When Shu's troops arrive not long afterward and he manages to successfully ambush and defeat them, he immediately sends a messenger to Cao Zhen, telling him to forget about their bet and maintain the utmost vigilance; when Cao Zhen reports that he hasn't seen any hint of Shu's forces, Sima Yi correctly infers that they're planning to launch a surprise attack, and immediately comes to Cao Zhen's aid, though not in time to keep him from suffering a serious defeat. After rescuing Cao Zhen, he doesn't reproach him for his incaution, but instead tries to get him to focus on salvaging the Wei forces' position, adding, "Let's have no more talk about our wager, but serve our kingdom with united devotion." This incident again demonstrates how completely he's put aside his ego, as well as a great deal of patience and forbearance - the last of which he initially offers to Wei's enemies as well, for the sake of trying to give the country a chance to recover from what, at that point, has been fifty years of nearly unabated civil war.
Not long after his defeat, Cao Zhen falls ill and dies, and Sima Yi ends up doing battle with Zhuge Liang again; when they are exchanging challenges, Sima Yi doesn't boast about how he's going to wipe Shu and Wu off the face of the earth, but instead, offers peace if Zhuge Liang is willing to accept it: "But you and your commanders can still save yourselves if you will reflect and alter your course, withdraw now and let our three separate kingdoms defend their borders in a settled order, thus sparing the common souls of our land the miseries of war." Then, rather than threatening to catch Zhuge Liang and execute him, he demands that, if he loses, he retire and return to his village, "under my guarantee of safety, of course." Zhuge Liang then proceeds to beat his capacity for mercy out of him, starting with the ensuing battle; in the initial clash between their forces, Sima Yi's battle formations are bested, so he brings his reserves into the fight, personally leading the charge (demonstrating that, while he doesn't relish bloodshed, his distaste for it stems from compassion rather than cowardice, and he's capable of holding his own in battle if need be). Again, he comes off the worst, and thereafter, he adopts considerably more cautious tactics in dealing with Zhuge Liang. This caution is cemented after, when Zhuge Liang is withdrawing his forces from the field at the end of this campaign (the fourth of his northern expeditions), Sima Yi's vanguard general, Zhang He, convinces him to undertake a pursuit (the incident in the novel differs from the historical version of the incident Wikipedia lists); in the process, however, Zhang He is ambushed and killed, leaving Sima Yi with almost no one he could rely on in framing a strategy to defeat Shu.
With two of his trusted colleagues - both of whom were his close friends as well - deceased, and with Zhuge Liang having finally succeeded in getting inside his head, the fifth and final of Zhuge Liang's northern campaigns proved to be the one that nearly broke Sima Yi (though, he got off comparatively lightly, given that it killed Zhuge Liang). While the first victory of this particular campaign went to Sima Yi, Zhuge Liang soon more than made up for his losses, finally solved Shu's supply problems (thus rendering useless Sima Yi's fallback strategy of waiting the enemy out), and took advantage of Sima Yi's policy of sending captured enemy troops home to feed him small victories in order to overcome his completely justified fears that Zhuge Liang would find a way to entrap him if he did anything other than defend - and then, once Sima Yi was confident enough to stage another offensive, Zhuge Liang came within a hairsbreadth of killing him and his two eldest sons with one of his infamous fire attacks. Unable to attack, Sima Yi returned to a strictly defensive posture despite Zhuge Liang's repeated insults and provocations (which weren't easy to shrug off - by this point in the novel, Zhuge Liang had killed three guys just by insulting them; I am not making this up) and the growing unrest of his own troops; the novel repeatedly mentions his despair at the situation, until he figured out that Zhuge Liang's health was failing.
Even once Zhuge Liang finally died, Sima Yi worried that news of his death was only another ruse, and pretty much shat a brick when Shu's army pretended it was; though he tried to laugh it all off and pretend that everything was fine now that Wei was safe, later incidents in the novel demonstrate that Wuzhangyuan continued to prey on his mind well after the fact. Though he had won, the nature of his victory did little to restore his confidence, and destroyed any capacity for compassion he had toward those who would disturb the land's peace. For example, three years afterward (and after the canon point I'm pulling him from, though I feel this bears mention anyway) he pushes his troops to the limit in putting down Gongsun Yuan's rebellion in Liaodong, and rather than offer him any kind of mercy, he drags out negotiations until Yuan's army is out of supplies, and once the enemy has lost the will to fight, isolates Yuan and executes him along with his entire family and all of his adherents.
As I mentioned above, Sima Yi's primary loyalty is seen to be to the good of his country, rather than the interests of his ruler - only natural, as hard as he has worked to make the state of Wei great, and given that he takes quiet pride in his accomplishments for its sake. Because of this and his considerable talents as a strategist and statesman, his relationship with Emperor Cao Rui hasn't been the best, as the latter finds his ability intimidating. For his part, Sima Yi is underwhelmed by the young emperor, and knows his own skill at managing the state is far better - and on some level, he still feels slighted by him for believing Zhuge Liang's ploy to get Sima Yi dismissed from office on suspicions of treason. However, following Sima Yi's steadfast service in defending against Shu, and due to Sima Yi's tremendous patience, relations between them are improving - and even if they weren't, going into rebellion is something that Sima Yi would never even begin to contemplate (as shown by the fact that, when he's accused of it, though he more than has the means to rebel, he sends his troops back to their garrisons and goes alone to get the situation sorted out with Cao Rui, knowing it could well get him killed). As I said, this is due in part to how important his service to Wei has been to him; however, it also stems from the memory of his close friendship with Cao Rui's father and Wei's first emperor, Cao Pi, as well as his unreserved admiration of Cao Pi's father, the famous warlord Cao Cao.
When he first entered the government, Sima Yi soon became friends with Cao Pi - though Cao Pi didn't live up to his father's brilliance as a statesman (no one in the era ever did; even Zhuge Liang failed to provide for his country's continued prosperity and survival after his death), Cao Pi was shrewd and tenacious, and made up for any shortcomings in his personal abilities by humbly accepting the wisdom of his advisors; he knew that if he was to compare with his father's reputation, he had to make use of every advantage at his disposal. Sima Yi admired his dedication, considered him the best choice to run the country when Cao Cao was gone, and played a role in persuading Cao Cao that he should designate Cao Pi as his heir; later on, he advised Cao Pi in forcing the last Han emperor's abdication. In turn, Cao Pi always seems enthusiastic about Sima Yi's plans and ideas in the scenes they share together, and when Cao Pi takes the field to lead an invasion against Wu, Sima Yi is the one he leaves running the government, despite the fact that Jia Xu, one of his oldest and most trusted advisors, is still alive at that point and might seem like the more natural choice. However, Cao Pi's confidence in Sima Yi is absolute, and if Cao Pi actually liked any of his blood siblings, it could be said that he treats Sima Yi like a brother.
If Cao Pi is his adoptive brother, Cao Cao is like a second father to Sima Yi; their meeting was one that changed Sima Yi's life and his outlook forever, and he admired Cao Cao from the very beginning. Originally, Sima Yi never intended to serve in the government; he saw no reason to sacrifice himself for a failing dynasty, especially when there were a surprising number of officials he considered at least as competent in affairs of the state as himself already doing that. He would rather have left the government work to his elder brother and remained at home to watch over his family, and retain at least some measure of control over his own destiny that way; perhaps his uncommonly thorough knowledge of astrology had given him some insight into the kind of suffering that lay ahead for him after he chained his fate to the country's. However, his first meeting with Cao Cao made a powerful impression on him. His attitude was far from that which had characterized most of the Han officials before him; while his brilliance, determination and magnetic personality made him an almost irresistible force, his own personal interest was a distant secondary concern when it came to how he ran his government. His primary concern was for rebuilding the country, and returning peace and stability to the lives of its citizens - and he had practical, concrete plans for how to accomplish just that. In him, Sima Yi found someone he could admire without reservation, and it was from Cao Cao that he learned what it meant to be devoted to an ideal that was bigger than oneself. For his own sake, Sima Yi had no ambition, but for Cao Cao, he wanted to conquer the entire realm - and Cao Cao valued him highly not only as an advisor, but on a personal level as well. For example, there is one scene wherein Liu Bei has just sent a message to the emperor, proclaiming that Cao Cao is a rebel and Liu Bei intends to destroy him. Cao Cao immediately goes into a rage, and prepares to muster his troops against Liu Bei, when Sima Yi checks him, and indicates that he has a better way in mind to deal with the situation; in other, similar scenes in the novel, this kind of thing has only been shown to make Cao Cao angrier, but this time, it completely turns his mood around (his reaction is translated by Moss Roberts as "Cao Cao asked amiably, 'Zhongda, what is your esteemed view?'").
However, despite his personal affection for Sima Yi, Cao Cao realized that the two of them were greatly alike, and that could lead to problems down the line; while he knew that Sima Yi's loyalty to him and to Cao Pi was absolute, he also knew that Sima Yi wasn't the type to suffer fools gladly (for example, he refused overtures of friendship from Cao Hong, whom Sima Yi considered an idiot and whose character he held in contempt, despite the fact that Cao Hong was a close friend of Cao Cao's), valued the common good higher than the whims of his ruler, and would chafe under a lord whose competence and ability he could not respect. Moreover, he saw that once Sima Yi reached a certain level of influence, the growth of his power would be difficult to check, as Sima Yi's immense talent meant that he would inevitably outperform the scope of his responsibilities, and his knack for recognizing, promoting, and winning the friendship of men of ability (and when it comes to whom he associates with, ability is all that matters to him - for example, he's close friends with Cao Zhen, whom several of the other figures in the novel regularly tease for being overweight, and he mentors Deng Ai, who is socially awkward due to a speech impediment; as for his ability to win people over, Guo Huai initially dislikes him, probably due to Sima Yi's lack of military experience when he first takes the field, but by the time of Wuzhangyuan, the two of them have become friends) meant that he would naturally fill the administration with his adherents. Cao Cao knew firsthand the dangers of any one faction becoming too dominant in the court - after all, he was puppeteering Emperor Xian at the time. Not trusting Cao Pi's successor to be competent enough to control Sima Yi, and not trusting Sima Yi to always be a better man than him, Cao Cao took the same precautions in dealing with him that he would against himself. Sima Yi understood his worries well, and throughout the novel, is shown to be consistently striving not to follow in Cao Cao's footsteps.
After his duty to his country, family is the most important thing in the world to Sima Yi, and the Sima clan as a whole is extremely close-knit. This is most readily apparent in Sima Yi's relationship with his two eldest sons, Shi and Zhao. When Sima Yi becomes one of the novel's central figures, his sons are always by his side; he relies on their judgment on numerous occasions during the war with Shu, and throughout the rest of the novel, they continue to be the only ones he is shown to trust implicitly.