Based on the erotic game by Grand Cru Bourgeois.

Angels and Demons like "Naruse Mio" have been fighting for a long time to see who is in charge. In order to bring the closure to this feud. an angel named Sera and a demon named Firika sent to the human world to complete this competition. The leaders of both angels and demons decided that both Angels and Demons had to compete for the love of a human named Yamato. Thus, Sera and Firika compete for Yamato's love and the supremacy of their entire race.

(Source: Nekopoi and Has Been Translated)