There’s one thing I don’t get. Why did Riku turn on Sora so easily, when later games show he loves him at least as much as Kairi?
So a few things that are really important to keep in mind when considering Riku’s actions:
1) He was only fifteen in the first Kingdom Hearts
2) He is incredibly insecure
Riku’s character arc has always been about proving himself. Consistently he has impossible standards set for him, either by others or himself. Think about this: When Terra came to the Destiny Islands, Riku was five years old. Five, and then he’s told that his entire world is only one part of a larger universe and is tasked with a weapon designed for protecting the balance of that universe. That is a lot to put on a literal five year old, one who already is thinking far too much about the future. (Strength to protect what matters. They’re not even in elementary school and Riku is already concerned with being strong enough to protect Sora, and later Kairi.)
So Riku grows up waiting for that day. He knows he will leave his entire world behind. He knows he will inherit a keyblade. Or, in his mind, the keyblade. He thinks that there is only one, and that he was the single person in the universe chosen to wield it. Of course he’s going to grow up with an incredibly skewed perception of himself. He is special, he is destined for greatness. By the time we reach Kingdom Hearts, Riku is cocky, narcissistic, and desperate to follow his destiny.
So through the ten years of believing he is unique in the universe, he also builds himself up as a protector for his friends. Kairi is a small, fragile girl in his eyes (and to be fair, she plays the part, but that’s a discussion by someone else). For Riku, she’s the perfect base for his persona. He protects her and impresses her the way boys and girls are supposed to, she’s the princess and he’s the knight. It’s more in an older brother sense than a romantic one, according to Kairi’s VA. We can peg the reason he pretends to be interested in her romantically as just messing with Sora.
Now for Sora. They’re best friends, for sure, but Riku has a weird way of showing it when we first meet him. They’re not equal at that point in his life. There’s a huge power imbalance, and that’s exactly how Riku wants it. His attitude towards Sora is called a downward comparison. He boosts his own confidence and self esteem by looking at Sora and assuring himself that he is stronger, faster, smarter, wiser. However, as he reveals in KH2, we know that he’s been quite jealous of Sora’s bright outlook and sociability, implying that despite Riku’s (frankly aggressive) projection of his own self, he still was unsatisfied with who he was.
The points of the last couple to paragraphs were to say this: Riku thought he was destined to protect the universe with the keyblade. Riku thought he was better than his friends, and needed Sora (and arguably Kairi) to be inferior to him. Riku thought his friends needed him.
Riku thought he was destined to protect the universe with the keyblade: Sora gets the keyblade.
Riku thought he was better than his friends: Sora shows he is perfectly capable (maybe even more so than Riku) on his own in Traverse Town.
Riku thought his friends needed him: Sora appears perfectly happy traveling with Donald and Goofy.
Can you see how quickly that resentment would build? To him, this is a betrayal. He’s looked after Sora his whole life and now Sora is throwing all that aside, on top of stealing his destiny. Combined with Maleficent manipulating him, the influence of darkness, and later Ansem’s hold, Riku is drowning in negativity. He’s fifteen years old, he’s lost his home, his friends, his destiny, and the easiest way to deal with that is to get angry. As far as he is concerned, Sora is his enemy now.
