Forever Blue

Gray Fullbuster, Fairy Tail's head of Security, is at his wits end when he gets put in charge of protecting Juvia Lockser, a former dancer on the run from her previous employer, Phantom Lord. Is Juvia truly innocent, or is she part of a larger scheme to infiltrate Fairy Tail? And why is her helpless dependence on Grey causing him to risk his own life to protect her?

Book 3 of the "Heart Shaped World" series.
While technically a series, each story is written to be stand-alone, so you don't have to read them all to enjoy.
That being said, if you do enjoy the alternate-universe in "Forever Blue" you may want to check out the others for more glimpses of the Gray and Juvia you've just met as well as many other favorite Fair Tail characters and couples.