Itsuka Tenma no Kuro Usagi

Japanese Title: いつか天魔の黒ウサギ

English Title: A Dark Rabbit Has Seven Lives

Chinese Title: 天魔黑兔

Plot
Kurogane Taito is a freshman in Miyasaka High. Ever since an injury to his leg (a snapped tendon) that prevented him from practicing karate, which he had excelled in since elementary school, he has always believed that he was an ordinary, regular guy. However, due to a promise exchanged with a beautiful Vampire (Most Ancient Sorcerer) Saitohimea nine years ago which he has forgotten about, he is in fact no longer ordinary. Nine years ago, Saitohimea injected him with a poison, which prevents him from dying as long as he doesn't die seven times within fifteen minutes. Shortly after his change, a boy named Kurenai Hinata attacked them with a demon and killed Taito six times in rapid succession. In order to prevent him from killing Taito outright, Saitohimea agreed to leave with Hinata and allow Taito's memory to be wiped. Subsequent to this, Saitohimea was experimented on and imprisoned in a dimension with no sound or light whatsoever, and Taito continued with his life as a normal human.

Taito has recently been having a recurring dream concerning Saitohimea, although he is unable to remember her name. After saving Andou Mirai from being hit by a truck he is himself killed, however due to his conditional immortality, he survives. After his body picks up and reattaches his head, he begins to remember more details about Saitohimea and eventually recalls her name, which allows Saitohimea to regain her powers and escape from her prison. By this point, Taito has realized that not only are his most recent injuries healed, but his previous leg injury is also healed. He is attacked by the Church but is able to survive and he makes his way to the playground where he and Saitohimea met previously, where they are reunited.

Along with unlikely allies, Kurenai Gekkou, Miyasaka High's student council president and genius who is bent on avenging his parents as well as Andou Mirai, Gekkou's cute lightning demon familiar, Taito and Saitohimea have to contend against Gekkou's younger twin brother, Kurenai Hinata, who seeks to resurrect the powerful Vampire (Most Ancient Sorcerer) Bahlskra. Unknown to them, their destinies were already woven and foretold in an ancient prophecy of epic proportions.

OVA
After taking a nap, Mirai is thirsty and Gekkō refuses to buy her a drink. She notices a strange-looking potion on the table and chugs it despite Yūichi previously telling her not to touch it. The potion causes the person who consumed it to swap souls (thus switching bodies) with anyone he or she comes in contact with. The main cast spend the day inadvertently swapping bodies due to Mirai's carelessness.