I have a new shamisen, and the skin is made of cat skin. The very light applique, used as a bachi pic guard, has started to peel away from the skin. How should I reapply it? I’ve been told that any kind of moisture is bad for the cat skin, so I don’t know what to use to get the guard back down.
Hello Masahiro, welcome to Bachido!! 
You can use a light amount of Elmer’s glue to reattach the bachi guard (bachigawa). Just a tiny amount won’t damage the skin. Put a small amount of Elmer’s under the bachigawa and press it down to the skin. Squeeze out any excess glue and wipe it off with a towel.
Cheers!
Kyle