Guardian Alien

Jana wants nothing more than to keep out of the headlines. Her prominent political family is buried under accusations that are entirely false, and she cannot afford another blow to their name. Then Cavin turns up. He was once the boy she called her invisible companion, and now he is a grown man in armor who warns that an alien invasion is close at hand. Jana has no choice but to bring him to her leader.

Cavin has not forgotten the girl who refused to believe he existed during his earlier stay on Earth. Now a decorated space marine, he is ready to gamble his own future to protect her. He must outmaneuver government agents, her grandfather's distrust, and the most feared assassin in the galaxy, all while her public image stays spotless. If the plan succeeds, the world is saved, and Cavin may also win the heart of the Earth woman he never forgot.

This is the first book in the Otherworldly Men prequel trilogy, which continues with Royal Recruit and Cyborg and the Single Mom. It is also connected to the Borderlands saga, whose main books are Warleader, Hunting the Warlord's Daughter, and Raider Born.