Hi, Today's pick is the Seinen Sci-fi Action manga "Tetsuwan Birdy (II)" about the interplanetary police officer Birdy Cephon that while on Earth chasing a criminal accidentally kills Tsutomu a High School boy.
Then Birdy is forced to share her body with Tsutomu so he can survive. Now they live with the same body either morphing in Tsutomu for his day to day life or in Birdy fighting Alien criminals.

Description:

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.