It's his loyalty quest. Zaeer doesn't have a recruitment quest. You literally just go to Omega and he's near the entrance, and he's like "sup bitches, Illusion Man hired me".
The character kind of seems shortchanged relative to the others, unfortunately. He's great, and I've been using him ever since I got him, but he has no recruitment quest like most of the others and you also can't engage in any dialogue with him on the ship (or much of anywhere else after you do his loyalty quest).
That hurts his character development a lot, especially when you look at how deeply you can delve into the backstories of your other crew members. There's a lot of potentially interesting stuff they could have done with Zaeer. Guess they ran out of time.