How would you describe Lily and Zach's relationship? What drew them together?

Zach visits on occasion, and seeing him causes Lily’s stomach to turn circles. Lily tells him that she considers him a friend, a boyfriend, a beekeeper, and a brother. She tells him what would happen if she were black, but Zach replies that there is no use in that kind of speculation. Instead they have to think about changing the world. Lily discovers something in him that has grown angry, and he fills his mind with complex race-relation issues. Lily wishes they could return back to the simple conversations they had before.