Quote Originally Posted by Necronopticous
That will probably be revealed, along with the rest of the plot, when the game comes out. I would guess it's actually because he's cool looking. Vaan is all like, "Hey, man. You're cool looking, I didn't expect that from a murderer, you know? How about you join us!"
OMG YEAH HES TEH KEWLEST
Really though, I'm sure the game will provide us with interesting circumstances to him joining the party.