Kairi is quite an important character to making this game flow and creating the Riku we know. It was her that Riku was trying to find and wanted to save her. He was disappointed that he didn't get the keyblade and thought Sora didn't care for Kairi and was just messing around with the keyblade. When Ansem possessed Riku, he tried to open Kairi's heart through Sora. Riku, being brainwashed by Malificent, thought that opening the final heart or door he would be able to find some cure for Kairi. And to get through that door, he would need the keyblade that Sora had. He needed to be stronger so he joined darkness for the keyblade just to save Kairi. So therefore, kairi is the reason that Riku is evil. ...