Sequence (2002)

Blurb:
As punishment for being late to class, Kanata has to ring a bell in an old, deserted church near his high school. When the floor breaks, Kanata falls into a vast room, where he discovers a coffin--and inside, the body of a vampire. The vampire rises, killing Kanata. But Kanata’s soul is saved, and when he later meets the vampire, they realize that they are bound to each other through a magic spell: If Kanata dies, the vampire dies, too!

Source: www.anisearch.com/manga/4748