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.