Riku's eyes widened at the fangs, and then the whole exchange with the knife and telling Sora to cut himself—what?

"Whoa, wait a second."

He took a step forward, gaze narrowing as he looked at Yuuki. Forgive him, he was still playing catch-up here, but pointy teeth, calling himself a monster (and a dead one, at that), possibly being tempted to do something if blood was spilled...

"When you say you're a monster...what, exactly, do you mean?"