Well, I haven't used him myself, which is why I'm going to be trying him out in a game later today, but considering how he interacts with and makes burning tokens, I think he'll fit in a Kaeris crew just great.
He gives out burning tokens when he hits in melee, has a trigger that allows him to do +1 Dg when he hits in melee for every burning token the opponent has on then (which, in a Kaeris list can easily grow to be huge), and has a (1) spell that is a Dg2 4 which also hands out burning tokens, and deals 1Wd and a burning token whenever he's hit in melee. Also he has Ablaze, making him immune to burning tokens.
So yeah, at least on paper, it looks like he's totally worth the seven soulstones to hire him.