Callan is very upset. This is the first (and Callan hopes the only) time Mizuki or anyone will see him showing an emotion that isn't "silly" or "murder". The bubbled letter winds up floating under the surface of the water, still bubbled but out of sight now. It's going into the hoard of stuff at the bottom of the lake.

He takes a breath and shakes what looks like a head side to side. The Creature yaps in his ear the entire time, and Callan ignores it, giving Mizuki no sign that he's hearing anything.

"You've been kind to me, little sea creature. Why?"