18+ Oni Uta.: Oni ga Kitarite, Amaesasero to Notamouta.

There is a small village located far from the town. The main character is Akito, who lives at a shrine. His current concern is that he has only few friends. He has an older sister, Koharu, who excessively relies on him, and an old friend, Ayako, whom he is in love. One day, he encounters an offering box burglar and is killed by him...

It looks like his life is finished, but Himeka, a demon princess, suddenly appears and saves him. Himeka loses almost all of her power for that and orders him to take care of her for a while. Since Himeka actively approaches Akito, Koharu gets jealous and conflicts with Himeka. Like this, Akito gradually gets involved in an epic battle between Himeka and Koharu...

(Source: ErogeShop)