Just more work on fleshing out Antipode 'verse. Yup.
Sora: First and foremost, Sora is Riku's best friend. He has been ever since they were kids. They've been through a lot together, be it silly little 'adventures' when they were little to building a raft with Kairi... and then everything that happened after the storm.
While he didn't necessarily show it at first, Riku constantly wondered about Sora when he first got the Keyblade. Though at first he brushed it off, figuring he was off in another world, everything started to change once they met in Traverse Town. He was okay-- heck, he was the same old sap from the islands, but he'd made new friends. Suspicious ones, he'd later find, once Leon explained the truth about Maleficent.
And then people kept on mentioning Sora's presence, even as early as Wonderland. And every time Riku would meet up with Sora he'd reveal more and more, like the truth about Kairi, all while pleading for Riku to help. For whatever reasons, Sora genuinely believed Maleficent and the others were his friends, something which sickened and frustrated Riku to no end, even if he wasn't terribly surprised. By Hollow Bastion it'd gotten to a point where they both had no choice except to fight, something which neither of them desired.
It cost him his Keyblade temporarily, in fact, for Riku became too fearful that their friendship would be destroyed, or that something terrible would happen to Sora. In the end, though, Riku pressed on because he wasn't about to give up on Sora-- and besides, who else would knock sense into him?
Luckily, it didn't take much longer for Sora to come to his senses, and the two even joined forces with Donald and Goofy to take down Maleficent. But perhaps the biggest turning point for Riku and Sora's relationship was what occurred right after.
And why was it so important? Simply because Sora had no other choice but to use the Dark Keyblade to free Kairi's heart, something which would supposedly destroy him in the process. It left Riku in a tense situation, stuck between the fates of his two best friends... and in the end Sora sacrificed himself.
At first Riku doubted he was alive, but after some reassurance from his friends, Sora became his new object of motivation. He even promised Kairi that they wouldn't return home until they found Sora-- or whatever was left of him. It's because of this resolve that he left the door to Kingdom Hearts wide open and also why he gains such negative feelings towards his Replica and the Organization. Unaware of Sora's Heartless, Riku believes his Nobody to be the only part of him that isn't a mindless creature of darkness, and said part doesn't even have accurate memories.
In short, from his current point in the story, Sora is a sensitive spot for Riku. Incredibly important even now, of course, but also an object of a bit of guilt and worry.
Kairi: Like Sora, Kairi is incredibly important to Riku, albeit in different ways. Much like Sora acts as an inspiration now, Kairi really was the person who really sparked his interest in other worlds. Without her, he might not have thought of the raft, and to that end he's grateful that she came to the islands.
But there's so much more to his relationship with her. He doesn't just see Kairi as proof of other worlds; Riku genuinely sees her as a friend... to put it mildly.
Unlike most girls, Kairi can really keep up with him and Sora. She's spunky, fun, and her somewhat tomboyish nature is something he really admires. Riku really enjoys snarking at her because she'll tease right back, and while he goes a lot easier on her than Sora, he doesn't mind rough housing with her, either.
Over the years, Riku's developed a bit of a crush on her, in fact, although he certainly knows how to keep it to himself. While he often teases Sora about her, the truth is that his bluffs about paopu fruits have some truth to them. These feelings are something he practically admitted to Donald and Goofy after Halloween Town, after hearing the truth about Kairi from Sora. While not exactly direct confirmation, when Goofy asked if she was his special someone he stated that he wished that were the case.
During the time that she was asleep Riku, just like Sora, was determined to save her, even if he didn't know how. He'd even considered kidnapping Pinocchio from Traverse Town to see if he held any answers, although he changed his mind out of better judgment.
And of course, once Sora gave up his heart, Riku tried his best to be there for her, even going so far as to making a promise to find Sora and also letting her travel with Donald, Goofy, and himself. Even then, Riku's less inclined to admit any feelings to her (outside of the DR) because he's worried that it would come across as selfish, especially with Sora supposedly gone. He's good with being one of two best friends for her, although deep down he wishes it could be more than that.
Donald: At first...Donald and Riku didn't get along much, if at all. Riku generally found Donald to be far too up tight and strict about little things such as meddling and the "mission", while he just wanted to check out other worlds and maybe track down Sora and Kairi along the way. Many times Riku would walk off on him and Goofy or blatantly ignore orders, and the only times they'd really work together would be during times of crisis, such as heartless battles.
That all changed after Pleasure Island. Once Riku finally came to understand how crucial his role was in their mission and the importance of the Keyblade, Riku immediately changed his plans and genuinely chose to work alongside Donald and Goofy. Though they still argued-- and still do even now, Riku's got a strange respect for Donald. When his temper isn't in the way, he's a pretty good friend, even if he's not as close to Riku's heart as Sora and Kairi.
Goofy: Like Donald, Riku didn't care for Goofy at first, although he did prefer him to the duck. Mostly, he found Goofy to be kind of a moron and much too friendly. His silly antics and comparisons to his son Max didn't really help, either. Yet, somehow, Riku did appreciate the fact that Goofy had a bit more faith in him than Donald and never really reprimanded him. There was something...oddly parental about him, which makes sense, since Goofy's a dad.
Needless to say, he's on much better terms with Goofy now and actually does consider him to be a friend. He's not exactly the brightest bulb out there, but Goofy does have his moments of genuine insight and concern, kind of like Sora, in a way. To that end, he's most definitely a close friend, but, like Donald, no where near the level of Sora and Kairi.
XIII // "Sora": So, so complex. Whenever he sees XIII, Riku can't help but feel a rush of different emotions. First of all, there's the fact that he's Sora's Nobody. To that end, Riku feels obligated to knock some sense into him and bring him back home. At the present moment, Riku really doesn't see XIII as his own person, but rather an extension of Sora-- what's left of his best friend after losing his heart.
The fact that his memories are altered hurts, however, no matter how close to Sora this Nobody may be. Not only does XIII blame him for his loss of heart, but he also doesn't remember Kairi, which makes things especially worse because he'd promised Kairi to bring Sora back with them, not to mention how the three of them have always maintained a special bond toward one another. It's upsetting to see "Sora" forget, especially when it hurts both him and Kairi, and it's also made Riku determined to find out who did this to him and undo his train wreck of a memory.
And then there's...relief. In a way, Riku's grateful that XIII exists, because it shows that Sora isn't dead and gone; a part of him's still out there, and heart or not heart, Riku's determined to set things right. Then and only then will he close the door to Kingdom Hearts... well, and after he defeats the Organization, naturally.
Riku Replica: Here's a guy Riku isn't sure about. A part of him really, really wants to hate his clone for "taking away" his best friend and causing all sorts of trouble for him. After all, he goes around sabotaging worlds he tried his hardest to save, and all to tarnish his name. Riku doesn't even understand why; if the Replica thinks himself to be the real Riku, then why would he go so far as to turn against 'friends' just to get revenge?
It sickens him, because Riku believes that he'd never stoop to such a low, even if a Replica did steal his identity and his best friend's heart. His clone seems like a twisted caricature of himself, really.
And then there's the fact that his Replica can't really help it. His memories seem authentic, and he honestly believes that XIII is his best friend and that Riku-- in his mind, the "fake"-- caused him to become a Nobody. His "friends" replaced him with Riku in said memories, and if Riku were in his position, then he'd definitely feel horrible. So, to an extent, he sympathizes with the Replica.
Regardless, though, Riku won't give up on his best friend all because of some copy. The Replica is a threat, not just to his friendship, but to every world he trashes with his Keyblade and the Heartless. He's not about to let that slide.
DiZ: Suspicious. Absolutely suspicious. Riku doesn't trust DiZ as far as he could throw him, but for now he and his friends have decided to work alongside him. After all, he's provided them with a place to stay and even a new Gummi Ship so that they may resume travel across the worlds. Not only that, but DiZ is smart, and he demonstrates a great knowledge of the Organization and current events.
It's more that DiZ seems a little too into destroying the Organization and Nobodies. While they seem to be a threat, Riku suspects there's more to it than the guy lets on. Plus there's the basement of the mansion, which appears forbidden to he and his friends.
All in all, DiZ is an enigma. An enigma that Riku can't help but watch with a careful eye.
Ansem: Riku... did not like Ansem. Not at all. After all, it was Ansem who manipulated him into opening the door to the Islands during his fit of jealousy, and it was Ansem who taunted him constantly about Sora and how it was his fault that he sacrificed himself all because of the incident on the islands. Ansem manipulated Riku, tried to possess him, and, above all else, really was the root of the conflict in the first arc of the story. Maleficent and the others were vital tools at best.
His silver tongue and cleverness made him intimidating as heck, although Riku kept any and all intimidation to himself. His obsession with darkness? Riku was less concerned about, although he found it annoying and kind of idiotic. It wasn't due to light being superior or anything like that-- in the end, Ansem got what he deserved, and that's all there is to it. He has bigger fish to fry now, so for now any Ansem-based concerns are no longer a priority.