Birdy The Mighty summary:

Tsutomu Senkawa is an average high school kid who is busy studying for his senior entrance exams. One day, Tsutomu stumbles into a man who is on the run from Space Federation Officer Birdy Cephon Altera. The man throws Tsutomu at Birdy, who kills him only to realize her blunder right after. In order to save his life, Birdy and Tsutomu's body merge into one, and they must switch back and forth between being a high school student and being a Federation officer on the hunt for interplanetary criminals.