Spoiler! Highlight to read!
In the anime Finn Fish was brainwashed by the devil, and became a Da-Tenshi against her will. However, in the manga version the story is more complicated but she became a Da-Tenshi because she was punished by Kami-sama to step through a portal that would completely eliminate her. However she had made a promise to her friend Access Time before she left for the human world, that she would come back to hear what he wanted to tell her. Before stepping into the portal the devil offered Finn a chance to not be erased and she took it for the sole reason that she wanted to see Access Time again to keep her promise.