Angels and demons had fought over a long time to see who reign supreme. In order to bring closure to this feud. An Angel named Sera and a demon named Firika were sent to the human world to finish this competition. The leader of both angels and demons decided that both the angel and demon must contend for the love of a human named Yamato. Thus, Sera and Firika compete to for Yamato's love and the supremacy of their entire race.

(Source: Bilibili)