Reth is a fairy He is part of the bitchy ( mostly and perceived to be evil ) Court.
Reth can use magic, which enables him to use the Faerie Paths. When his full name is used, he must do what he is told to do but his name (Lorethan) . Reth is a member of the Seelie court and tries to do good, but has a altered Faerie version of it, and he and Evie have many conflicts. He is often perceived as a douche bag.
Reth takes an interest in Evie , and he attempts to give her energy (part of his soul). He and Evie were in some sort of relationship, but Evie and him broke up because after a werewolf tried to hurt Evie, Reth killed it mercilessly and Evie was terrified of him.