Luke is a half-blood from the Deity Island Covenant. He is one of Alex's closest friends and a skilled Sentinel. He first appears in Half-Blood when he whistles at Aiden while he's training Alex. In the first mention of him, it's revealed that he came out as gay while Alex was in the mortal world.
Sometime between the end of Pure and the beginning of Deity, he started a relationship with Deacon St. Delphi. Alex almost catches them in the sensory deprivation room and again in the library with Aiden. In Deity, he appears more often and becomes one of the main characters as he is part of the group that flees the Covenant after Seth killed the Council.
In and after Apollyon, he and Deacon are somewhat public about their relationship. In the companion series, Titan, they seem to be completely out now that pures and halfs are equal.
Luke is described as a funny but serious person and a fantastic fighter. Seth observed that even though Luke quit being a Sentinel after the war, he never stopped being one. He cares deeply about his loved ones, especially Deacon, and would do anything to protect them.