Who is Riku in Kingdom Hearts

He is a close friend of Sora and Kairi, he gets sucked into the darkness as a bad guy, Sora fights him in Hollow Bastion and at the end, when Sora beats Ansem, he goes into the Door To Darkness with Mickey. In the second game, he comes back with the image of Ansem, while turning back to normal when Ansem destroys Kingdom Hearts with his machine. At the end he fights with Sora at the last bosses, like the guys in Organization XIII and Xemnas, and the guys you fight at the end with the door. He eventually goes back home in Destiny Island with Sora where Kairi, Donald, Goofy, and Mickey was waiting