Birdy the Mighty (2003)

Birdy Cephon used to be a butt-kicking intergalactic Federation investigator who took down bad guys where they stand; Tsutomu Senkawa used to be a normal student with shy tendencies and a penchant for exploring abandoned houses. However, when Birdy accidentally killed Tsutomu while trying to apprehend a famous criminal on Earth, Geega, both of their lives changed forever! Now, while Tsutomu's body is reconstructed in a faraway place, the two must share the same body – Tsutomu controlling it by day, and Birdy taking over when she spots a criminal in need of tough justice. All Tsutomu wants is to live a normal life; can he manage to cope with Birdy's enormous eating habits, intense muscle use, and easily irritable attitude?