Riku is a well-practiced fighter. He's gone from the best kid at the wooden swords on Destiny Islands to the traitor with a keyblade on the worlds to the one who attempts to amend for his past. He trains every day, both by himself and with DiZ's help, and when he's lucky, occasionally he gets to play-fight with Mickey. Those are the best times of all.
At many times, he's participating in actual fights. Against Heartless or a low class Nobody, it doesn't matter, he'll pick the fight and battle it. He's got to get rid of the grudge so it doesn't hurt anyone else. It doesn't really matter if it hurts him, yaknow?
Usually, he's pretty lucky. Sometimes, he's not so lucky. This is one of those times. He got swarmed by Heartless, and although Yuffie managed to pick them off at the end, he'd hit the ground hard and smacked up his head pretty badly. In fact, he happens to be unconscious right now. So why are we even writing about him?
Because even an unconscious mind can operate, and his can operate in the most special ways of all. It can reach out and touch the most important people to him, and sometimes, even cross into their own minds for a safe haven...