What does the bond do?

You get a cute little chibi portrait of him on the keepsakes page, that's not nothing!

Six of the NPCs provide an... extra thing... in the locked lower slots when you get the bond. Charon isn't one of them. Most of the others provide extra dialogue and story advancement, acheivements, whatnot; Charon also isn't one of those. He's a bit underwhelming as a first bond, I agree, but most of the others are better.