An Intergalactic Officer, Birdy has the appearance of a human when, in fact, she is an alien. While she is giving chase to a interplanetary criminal on Earth, she accidentaly kills a young boy who was in the wrong place at the wrong time. As her punishment, Birdy has to share her body with his, preserving his life. So she has to deal with the boy, whose name is Tsutomu, along with his family while still doing her job.

(Source: Wikipedia)

Spoiler from ep. 12 of Season two.

Birdy is actually a successful creation of Ikushiora.