Tetsuwan Birdy

While pursuing a criminal, the pretty but reckless interplanetary police officer Birdy Cephon Altirra accidentally kills Tsutomu, an earthling high school student. To make amends, Birdy is forced to share her own body with Tsutomu's spirit so that he can continue his own life. At first, Tsutomu knows nothing about his situation but soon discovers that his body is starting to take certain actions without his mind ordering it. In times of action, Birdy takes full control of their shared body, transforming it back into her original female form.

(Source: MU)