I don't usually cut my essays, but because this one is going to be essentially one huge spoiler for Kastor's canon, I will. In summary, a ramble about how having Mikah around essentially changes how Kas deals with camp and the people in it, mostly so I have things straight in my head.
So Kastor Auberlane's life really kinda sucks. He was born a very obvious bastard, a milk-white child in a tribe of people whose natural skin color is likely somewhere on the deeper end of the asian spectrum. He's rumored to be the spawn of a demon. He was pretty much a feral child who ran away to live out on the Sei (which seems to be somewhat similar to the Mongolian steppes) on his own before he was ten years old. Then he as discovered to be a feahar, or a berserker, an uncontrollable mental condition where he loses control of his actions whenever he feels sufficiently threatened. As a teenager, he was chosen because of his feralness to be the consort of the queen, married off, and suddenly thrust into the position of consort. (Did I mention that he's a homosexual? or is that a given when apping from a Jesse canon?) Then, when the child he fathered was born crippled, it was declared that his demon blood had cursed the infant and he was banished from his home. He found work in an adjoining nation as a thief and hired killer. Then he took a job from Mikah, and managed to fall in love with a Mara. This is a pretty stupid thing to do, as Mara aren't generally given to loving anyone but themselves. Mikah, of course, broke his heart and disappeared for a year before waltzing back into Kas' life and demanding he go on an insane adventure to save the world. Things only got worse from there. Much, much worse.
But the important part? Mikah's payment for the job he offered Kastor was Kas' heart's desire. This is a man who has lost his home, his power, and his family. He has never met his son, and believes himself responsible for the fact that the boy is crippled. He has a condition that makes him an uncontrolled killer.
His heart's desire? Mikah. Even a year later, heartbroken and angry and pissy as a jilted woman, Kastor's greatest desire is Mikah's love. There are other things he wants, of course, but Mikah is at the heart of everything.
On his own, Kastor is pretty emotionless, outwardly. In general, he does poorly with people. He's a good listener but unwilling to talk about himself; he holds people at a very obvious arm's length; he cannot deal with crowds or being the center of attention. He makes no effort to get to know people or emotionally reach out on his own. Essentially, Kas has to be topped into any sort of friendly interaction. As a foreigner (and a member of a people who are looked on as savages) in any place he's allowed to go and an outcast in his homeland, he's trained himself to assume people don't want it. Internally, he's extremely passionate, but that doesn't reach outside his head.
Then there's Mikah. Mikah drives Kastor insane. He's loud and bright and obtuse, simultaneously childlike and patronizing. He drags Kas on insane quests and mocks him the entire way. Mikah can push every single one of Kastor's buttons, and he does. But with the exception of Charis, his son, Mikah is the only person we ever really see Kas be passionate toward or about. They are possibly the only people he really loves.
Now, Kastor is something of a brat. He can be extremely self-absorbed, and in most interpersonal reactions considers primarily how things affect him, not others. He usually drives Mikah at least as crazy as the Mara drives him. Mikah drags Kastor (and several others) off on a quest that has the side effect of saving the world, but the reason Mikah does it is to save Stiaan, his 'brother'. The only way to do this, unfortunately, is for Mikah to sacrifice himself to save Stiaan. Kas is not the happiest about this. But he eventually goes along with this plan, to the point of being the primary distraction so that Mikah can do what he has to do. The rest of the party does it to save the world; Kastor does it because Mikah wants to save his brother enough to die for him. It's not about saving the world, it's about making Mikah happy. He gives up his heart's desire because that's what his heart's desire wants most.
Kas takes a long time to even start to get over Mikah's death in any significant way. But of course, since it's Kastor, things can only get worse. By the end of the second book, a whole lot of stuff has gone down, and circumstances of all sorts have basically forced Kas to be Sad and Alone. In fact, the only person he knows that he can hang out with is Stiaan, and that's only because Stiaan is immortal.
That's all very well and good, you say, but what the hell does it have to do with anything?
Up till now, Kas has been very hard to play. He is the type of character who only forms relationships when he's topped into them, and that's not something that can be easily accomplished in camp since it's hard to be proactive about. More than that, he didn't want to form relationships, because all he really wanted to do was leave. As a 'bad shit' magnet, making friends was really just likely to put them in danger somehow. He was very self-controlled, very distant and unavailable. But with Mikah around, that will all likely change.
Mikah rubs Kastor raw. This was true when Kas thought Mikah didn't love him, truer when Kas knew he did, and even more true now that Mikah is back from the dead, however temporarily. The second Mikah walked back into his life, going home fell off the top of the priority list. He knows he's needed and will have to go, but being driven bonkers by Mikah ranks a lot higher than being chased around by demons and pushed around by deities.
But most importantly to his playability? Mikah makes Kas manifest emotions that aren't :|, :/, or >|. He gets Kastor involved in the world. Will he blossom into a social butterfly? pfffft. Will he suddenly be open to deep, emotionally charged relationships? Maybe, but probably not so much. What he will be is more open to interaction, even if a good deal of it comes from chasing after his completely insane lover.
tl;dr: having Mikah in camp changes Kas for the better because he opens Kas up to having manifest emotions, even if a lot of the emotion that manifests looks like rampant bitchiness. He'll be more open to interaction and actually forming any kind of relationship with other people now that Mikah's around.