So, as pretty much everyone and their dog knows, when you beat CoM, you get to play as Riku through "Reverse/Rebirth". Re:CoM is no different, although with animations and more voices than just a laugh or a pained grunt, there are some things to be noted nonetheless.
As in CoM, Riku moves a lot faster than Sora, jumps higher, and does more damage in combat. The downside to this being that he can't add cards to his deck, rearrange his deck at all, or hold onto cards from one world to the next. Dark Mode, at least, compensates some for this -- although it's worth noting that, in both games, you have no choice about going into Dark Mode; it just happens when enough dark points are accumulated. Poor Riku.
1.) The way Riku runs is adorable. You can tell the big feet are kind of getting in his way -- flop-flop-flop.
2.) What Riku does instead of his dodge roll, which one game FAQ for the original CoM called the "dodge jump". He appears to roll forward, catch his weight on his hands, and then do a handspring. He doesn't seem to get quite as far away from his enemy as Sora, but that's probably just an illusion created by the differing animation.
3.) Riku's cries of "Ousama" so summon his friend card once he has it and "Jama na!" during his duel...just kind of awesome, even though the former shouldn't be. (Ousama is "King", or, in this context, "Your majesty".) It just somehow is.
The last one comes from a new movie, called "Duel". I appreciate this feature and kind of wish Sora had had it. ("Ansem" teaches you how when you first encounter him face-to-face, by the way, before giving you dark powers.)
Duel works like so: When you and your opponent have matching cards, a triangle command will pop up. The action around you then "pauses" and you and your opponent attempt to do a tie-breaker. Break three of your opponent's cards before the timer runs out, and Riku will do a very powerful move that does a lot of damage to the surrounding enemies. Almost always, I find that my opponent is killed and when I regain control the field is littered with EXP prizes from other fallen foes. It's great for clearing a room.
By the way, "Jama na!" is a shortening of the ritual phrase "O-jama shimasu," which is what you say when entering someone else's home. The equivalent would be something like, "Pardon my intrusion." (Axel also likes to use this phrase, and says it when he interrupts Vexen and Sora's conversation, and again when he walks in on Marluxia threatening Namine. I love things like this, so much.)
Poor Sora. His friends just suck compared to Riku's friend. Stunning all my enemies and healing me you say? Reliably? Can I just replace Donald with you, please? Thanks!