Riku sacrificed so much just to get Kairi’s heart back. Even if it meant falling to darkness. He would do no matter what just to see her back to normal, even if he had to fight his best friend. He regretted what he did and left Sora to take care of her instead by the end of the game. Going from trusting Sora, to being jealous of Sora, wanting to get rid of Sora, then back to trusting him as a form of guilt and regret. All of this was caused due to the three of them being friends.
(Source: musicalsharpie)