Who is Ace? What happened between him, Scout, and Annie?

Ace is Scout's drug dealer, whose re-appearance generates additional tensions in Scout's already fractured relationship with Annie. Scout owes Ace a lot of money, and he tries to get Annie to have sex with him as a means of paying off his drug debt. Naturally, Annie is horrified at being used in this way and refuses to go with Ace. As a hopeless junkie, Scout is utterly indifferent to Annie's feelings; all he really cares about is getting his next fix.
Annie's been trying her best to stay positive, hoping against hope that a change of scenery would help Scout snap out of his addiction. But it's already apparent to her that moving to Oregon hasn't made the slightest bit of difference; Scout's still the same as he ever was. Annie, however, has changed. Ace's unwelcome appearance has forced her to take stock of her life. For the first time, she now seriously contemplates a future without Scout.