We will probably wait to talk about Tanner until you know him a little better. The conversation would be kind of useless if we don't exactly get the full context of the situation. But we will talk about him, I promise.
Okay. Here's how I set up Ethan's entry into the FBI. We can change it, but I kind of set up Ethan and Tanner's interactions around it, so it is moderately important. After Ethan convinces everyone in the FBI of his innocence (Except Tanner (-; ) Ethan leaves. Bree is set back on the task she had before this whole fiasco with chasing him down; trying to find evidence against a mob boss that has a strong presence on the east coast. Without Victor she has absolutely no leads. There is nowhere for her to go. She starts thinking about Ethan then, and what she thinks she knows he can do, and eventually she concludes that she has no choice but to track him down and guilt him into aiding her, because he does show honest remorse for Victor's death. Once she guilts him into it, they start working together in secret (because the person Ethan is pretending to be has no reason or ability to aid Bree) until Bree gets her first big, but completely unprecedented, break in the case. That's when Tanner is going to get mixed up in it, and it is going to get fun and complicated.
How does all that sound to you? I don't have any specifics on the case, just a general circumstance that can nicely guide it.