Shinjuku Badass Babies

One day, Kenichi, a 15-year-old boy living in Kabukicho, meets Maba, a 15-year-old girl who is also a hitman. Maba, who has a prosthetic right hand and left leg, was once a member of a certain organization and was searching for the person who killed their boss. In exchange for becoming Maba's arms and legs, Kenichi makes a "revenge contract" with her to avenge the death of his murdered father.

(Source: Natalie, translated)