There wasn't much on him, in any case, but after putting together what I found and what I already know...

Dark is an art spirit. He steals other art spirits to put them to rest. This means, Dark used to be a piece of art. Krad... his name is Dark spelled backwards, don't think I didn't notice, so they're related somehow. Most likely as... a piece of art. Or a former piece of art, in any case.