Kaibutsu Oujo S01E01 - Princess Resurrection

Hiro Hiyorimi passes by a mansion in the care of his sister, though he believes he is lost. As he wanders into the shopping district, he saves a mysterious girl from being crushed by falling construction beams, sacrificing his life in the process. As a token of her appreciation, the girl, later recognized as Hime, revives Hiro after he lays dead on the hospital bed. He runs back to the mansion, only to see Hime surrounded by a pack of werewolves led by Lobo Wildman. Hime manages to kill Wildman, thanks to Hiro's intervention. Because Hime has given Hiro immortal life, he is now bound to be her servant forever.