Title: Tetsuwan Birdy
English title: Birdy the Mighty

Tsutomu, an average middle school kid busy studying for the final exams to enter high school runs into a man running from someone and gets caught up in the chase. In all the confusion he gets accidentally killed. Luckily Birdy (an interplanetary agent) knows a way to save him, unfortunately that means joining bodies to become one. So now he is stuck with this officer and along for the ride capturing criminals and saving lives.

(Source: ANN)