He can pick locks for free, that sums up the good part of him. He starts off weak and is unable to promote so his skill always stay low. You’re better off with the other thief.
Edit: Actually, that’s in path of radiance. But yeah, he’s still an XP vacuum in Radiant Dawn, don’t use him, just keep him in a corner and level up better units. Volke is just way better and he starts up as an assasin which is great. However, you could probably go through most of the game only with Ike and Titania, it is a challenging game, but there are some amazing characters, so you can take it easy if you level up your team properly.