Guardian Alien by Susan Grant

To save Earth, she needed a GUARDIAN ALIEN

Jana has one goal: steer clear of scandal while outrageously false accusations pile up against her famous political family. But when her childhood “imaginary friend” Cavin shows up—all grown up—dressed in armor and warning of an imminent alien invasion, she has no choice but to take him to her leader.

Cavin has never forgotten the girl he met during his visit to Earth, the girl who didn’t believe he was real. And now, as a decorated space marine, he’ll risk his future to save her. That means staying one step ahead of government agents, Jana’s suspicious grandpa, and the galaxy’s most feared assassin—all while keeping her political reputation squeaky clean. If everything goes as planned, they’ll save the world—and he’ll also capture his favorite Earthling’s heart.

(based on the title Your Planet or Mine?)