Kaminari-sama no Koigoromo

Since childhood, there was a recurring dream she often had...

Akane, a girl with a rare blood type and whose parents were killed by a demon, was left utterly alone. Saved by her cousin, Soukou, she dedicated her life to serving him, offering him her blood. One stormy day, while being attacked by demons, Akane is saved by a demon called "Raiden." Terrified, as Soukou had warned her that Raiden"was a fearsome demon of thunder, but Raiden turned out to be quite different from what she expected...

(Source: Shounen Sirius, translated)