First a word about the characters, and their motivations going into book three:
Off page, at the end of the first volume (after the confrontation at the school) Aiden was in a really bad position. He considered just running, but he knew that would screw him over ultimately, so instead he did the opposite and used Amy to get a message to Derek.
He was allowed to keep his position within the Shades only AFTER going through a torturous process in which his loyalty and power were tested in some really horrendous ways (hurting people, hurting himself) and it really traumatized him. Amy legitimately helped him get himself together afterwards, which became the basis for an actual friendship, which developed into a romantic relationship. (Keeping in mind that Amy never knew that Aiden WASN'T romantic about her—because he'd been using her... She just saw a shift that took them deeper after the Derek thing)
