An angel gave away everything he had and became a girl, thinking that this will make his best friend fall for him. The said best friend is a female demon who is only interested with people of the same sex, and so the love given by the angel was not reciprocated even after turning into a girl. The reason for this is because the demon was already in love with a human girl. 

Filled with jealousy and rage, the now fallen angel swore to make the world a miserable place as her form of revenge towards the Gods who she blames for everything that happened to her.