I was trying to think of what a Riku and Kairi Limit would look like, and I have two ideas for it:
Riku grows a demon wing, while Kairi has a halo. Kairi keeps throwing her halo at enemies to trap them (think Wonder Woman's lasso), while Riku rips off the pointy ends of his demon wing to then throw at the enemies that Kairi has trapped for them, using them as something akin to daggers. I'm obviously going for demon and angel symbolism with them both here (that has been used for Riku and Kairi before, and even somewhat in limits they've had with Sora. But I'm trying to keep this different from their limits with Sora, obviously).
This is my favorite one: Riku and Kairi are first standing there with their Keyblades, looking at each other awkwardly. But then they look at each other more confidently, dismiss their Keyblades, and Riku attacks their enemies with his fists, while Kairi does so with her legs: the idea behind this one being that Riku and Kairi see each other as the boy and girl they are deep down, without all the bows and whistles (like when Kairi helps Sora to see that "Ansem Seeker of Darkness" was actually Riku). Which is why they would forget their Keyblades in this Limit and just fight like normal people.
