Sanemi is uppermoon 2, sharing the spot with his baby brother Genya. Their story is the same except They met earlier as Humans. Genya tried to Apologize but Sanemi fresh off his friend death and secretly hurting, yelled and cursed at Genya. Screaming how ungrateful he was and hoped he died. Sanemi then left Genya alone. Later the next week guilt been eating him up and Sanemi felt bad for what he said. He got a mission their and went to kill the demon but walked in on Genya armless, beaten and flayed body. Turns out Genya saved a girl from a demon but was accused of the deaths and was tortured as punishment. Sanemi, wracked with anger and guilt kidnapper the girl who accused him and the one who hurt Genya, tied them up and used his blood to lure a demon to them. The demon being uppermoon 1 Gyomei. Sanemi begged to have Genya saved, pleading for Genya to live. Gyomei felt bad for them so gave them both his blood. Sanemi remembers the past life in bits Genya doesn't.

As a demon Genya is a very demon who has morals he loves children and animals, often kidnapping them and give them toy and games. He will weep when he sees a child dead body and Sanemi let him cuddle it until it time to eat. His BDA is basically create plants from his blood and manipulate them. He can eat a human and temporarily gain there abilities 

Sanemi is extra aggressive, rarely having mercy but to appease his brother he spare the children and babies. His BDA is basically manipulating wind from his body. His blood can intoxicated a human and he lures them in a trap. 

They can fuse into each other but it mostly Sanemi whenever he in presence of other demons and It Genya whenever he in presence of humans.