Justin is a human who impersonated Lucifer.
Justin comes from Sherman Oaks. He went to Lux once, and told some girls that he owned the club. After realizing it helped him to get girls and free drinks easily, he decided to just roll with it. He slowly began ruining Lucifer's reputation and finances. Worse, at least to Lucifer, was that Justin was poor in bed and crier.
After Ronnie Hillman caught and delivered him to Lux, he still tried to continue his act and boldly claimed he was Lucifer. But after a little intimidation from Lucifer and Maze, he became scared and admitted his identity and intentions while begging Lucifer to not punish him.
In the end, Lucifer just let him go without injuries.