[@Inertia] Generally speaking the bending masters might know OF each other, but they shouldn't be particularly acquainted. From the looks of the characters so far as well it seems that most of them were traveling or solo agents anyway so it would probably feel more natural if they were to be meeting for the first time. The White Lotus is being especially secretive with this selection of agents because the Avatar is starting to mature nicely (like a fine cheese) and so San is more desperate than ever to get his hands on her. 

As for a character impressions sheet; it is coming! There will be a small journal like section for characters to write and update about how they feel about one another - first or third person is fine. I'm just waiting until the cast is final and so people don't need to start worrying about character relations etc only for someone to drop out/not get the role!