The original chosen one to wield the Keyblade, Riku chooses a darkness over light when he and his friends were attacked in Destiny Islands. His decision places him in Hollow Bastion where Maleficent influences him to turn on Sora in order to help Kairi.
It’s kind of sad that he made the wrong choice, or else he would have been the protagonist of the series. I like Riku’s character design better than Sora’s. This is still a very nice cosplay though! Great work!