Hellsing Kyouju no Omoide

Living an everyday life of despair, the girl Tokiko has closed her heart off. One day, she meets an old man who lives in the rumored haunted house of the neighborhood. The old man introduces himself as Professor Hellsing, and confesses that he is a vampire who possesses eternal life...! Tokiko is doubtful at first, but she offers the Professor help in pursuing his long-lost memories.

(Source: Kodansha, translated)