I haven't found a series to cross over with yet, so I'm considering an isekai setting...
He's Killua's younger brother, with white hair. (If he doesn't want to inherit the family business, don't force him.)
I probably won't expand too much on the Dark Continent, since the old geezer doesn't look like he's capable of turning in a manuscript.
Theme: The tearful tale of having to work for the family business at a young age because your brothers are so unreliable
