While they're still shrouded in mystery, I have a feeling in my heart that Ava will be my favourite no matter what. Besides, from what we've seen of her she seems like an utter sweetie.
My first impressions of all of them was that Ira was a bit of a jerk that meant well but had a terrible system of order, Aced was a stubborn, hotheaded fool that should have had more trust, Gula was a bit of a pampered child (but probably also the nicest next to Ava) and Invi was the uptight, strict mother of the lot of them. Ava just comes off as a more likeable character most of the time so far. That very well may change, however; it is to early to tell.