Kaminari-sama no Koigoromo

A girl named Akane, who has a rare blood type and lost her parents to a demon, was raised by her cousin Soukou, to whom she pledged her life and blood. One day, during an attack by demons, she is rescued by a demon known as "Raiden." Though Soukou had warned her that Raiden was a dangerous thunder demon, she finds that he is not what she anticipated.