Exactly my problem with him. He's too much of a scumbag for the others to associate with, in fact some of them directly oppose somebody like Therion in their ch.1 story. He's introduced as a lone wolf, his story backs it up, yet he immediately joins up with whoever just strolled into town. His travel motive is also the lamest and he's just there in battles. I don't like stealing from all the nice townsfolk either, will totally steal if Inquire / Scrutinize shows them to be a dickhead but that's it. Ugh, Therion. I hope he gets better.
With a set story order you could rationalise him using Tressa to get into the mansion, say he steals her diary or whatever and bribes her into getting him in, fully embrace his dickishness. That's the only tenuous link I can imagine.