The cruel former bully of Dylan Walker, Luke was one of the many kids who tormented Dylan throughout high school. He hated Dylan because he got to be himself, while Luke had to suppress his sexuality for years. He moved to Beacon Heights in an attempt to make things better between him and Dylan.
Dylan recognizes Luke as his high school tormentor and freaks out walking away. He explains to Ava and Caitlin that Luke made his life a living hell, before hurrying back to his apartment. He confided in Andrew, who agreed to confront Luke with him. Later the same day, Luke confronts Dylan and apologizes for the past behaviour of Luke towards him. He told Dylan that he was jealous that Dylan was out and proud, and that he couldn't come in terms with sexuality because he feared he would loose his loved ones. He tells him that he is out now and he has come to Beacon Heights to make things right with Dylan. Dylan, although happy that Luke has come out, doesn't forgive him for the way he ruined his high school experience.
Luke is the second character in the Pretty Little Liars universe to bully a fellow school student because of their feelings regarding their sexuality. The first to do this was Alison DiLaurentis, a now out bisexual woman, who teased Emily Fields for being gay.
He came out to his parents after high school and after seeing how well Dylan handled his sexuality.