Class-president Sakura Teshigawara tries to get the new transfer student Shosuke Samon to join a club or found his own as he is always alone in class. He got off the wrong foot with the class after claiming that his favorite Hobby was "summoning Demons".
Samon becomes interested in her after he learns of her being a "good person" including her image as an Angel. So he invites her to the roof to talk about something.
She get's exited at first, thinking he is about to confess his love until he tells her that he hates her because she is such a busy-body and a goody-two-shoes.
That would be the end of it, if not for a small detail: Samon can actually summon Demons and decides to get her to show her true colors by saddling her with Demons to tempt her into acting slothful or gluttonous for example. Watching greedy people getting corrupted by demons is a favorite past-time of his after all.
As the Demon world is pissed at Samon because he summons Demons so often, Assassins are sent after him and his perceived associates. He defends Teshigawara from one such attack, because he wants to witness her downfall and a violent death is too boring.