Destination Human (Orca Currents)

Chloe thinks of herself as a normal teenage girl—if there's any such thing—until a formless alien being inhabits her body. The being is named Welkin and claims to be a Universal. Welkin has entered Chloe's body as part of a school project. Chloe agrees to let this weirdo observe her life for three days as long as Welkin doesn't interfere. Welkin tries to respect the non-interference portion of the agreement. But Welkin's stream of alien commentary as Chloe deals with boys, her coach and math homework has a comic, and sometimes enlightening, impact on Chloe's life.