Trevor first appeared alongside his friends Tierno, Calem and Shauna in XY001. He was the one to give Lexi her Kalos Pokédex. He remained with her until she arrived at her Gym battle with Viola and then went on his travels with Tierno. He returned sporadically from this point, on his own journey.
As the series continued, and Team Flare's plans became clear, it was revealed that Trevor was infact Lysandre's son, and that he had been working for his father for some time. He had been on medication for a mental health issue for some years, and Lysandre had changed the medication, making Trevor medically insane. Because of this, he operated under his fathers wishes, including trying to kill all of his friends.
Trevor's finally appeared when he was dragged off in handcuffs, being arrested for his crimes. It was revealed later on by Calem that Trevor had been sent to a mental institution, after it was revealed his medication was restored and his mental state was fragile at the realisation of Tierno's death. He has not been seen since.