it's good to know royalty
Riku first encounters our beloved mouse King at the end of Kingdom Hearts
, when both are stuck in the realm of darkness. All we know about their interaction at that point is that together they helped seal the door. In the next game, the two get separated, and Riku ends up in the basement of Castle Oblivion. While going through his memories, Riku encounters Ansem once again. The evil menace continually tries to convince him that darkness is the way to go, but Mickey comes to the rescue several times. Although the King only projected an image of himself there until he found Castle Oblivion, he was able to chime in words of encouragement that made it possible for Riku to successfully resist the darkness and Ansem. At the end the King admits that he's curious about Riku's powers since they include both light and darkness. With that, they walk onwards side by side.
In Kingdom Hearts II, Riku leaves Mickey's side because he needed to deal with the Ansem in his heart on his own. They meet again after Riku has immersed himself in darkness and taken on the appearance of Ansem. Riku makes Mickey promise him that he won't tell Sora & Co. about what has happened to him. Later, Riku does some rescuing for a change and prevents Mickey from rushing over to the self-destructing cannon of doom that Ansem the Wise created and blew up with. At the end, the two enjoy a joyous reunion on the Destiny Islands.