Lydia Carlton, the daughter of Professor Carlton, is a seventeen year old fairy doctor. She has the rare power to see fairies, an ability she inherited from her mother, Aurora. Lydia also possesses green eyes, a trait which is associated with fairies. She possess slightly light brown ash like hair which she has a complex about since many call it "plain"; however, Edgar claims it is a "beautiful carmel color". She is independent and kind, and doesn't hesitate to help others. She ends up working for Edgar as his consultant on fairy-related matters after helping him retrieve the Merrow's Sword (the key to him being recognized as the Blue Knight Earl by the Queen of England). Even though she knows Edgar is constantly decieving her to a point where he of course brings harm but not only that almost sacrifices her life. Her goal is to become a full-fledged fairy doctor, and she frequently places herself in danger willingly when it comes to anything to do with problems between humans and fairies.
She appears to have some self-esteem issues, referring to her own hair as "rust-colored" and comparing herself to her mother, who was a woman of considerable beauty. Once, in an outburst, she confesses to Paul that she thinks that she isn't cute at all, and that's part of the reason why she tries so hard to help in other ways by working hard.
Because of past difficulties, Lydia dislikes the idea of falling in love and even seems to be afraid of it. When she was a child, she was given a love letter by a boy who, as it turns out later, was dared to do so by his friends, crushing her. She also used a "spell" to ward off Kelpie so that he wouldn't come near her, as his aggressive advances were troublesome to her. At first, she also rejects Edgar's advances and brushes them off as a joke, but as she falls in love with him, she demonstrates substantially less distrust of his confessions of love.
Lydia currently is considered Edgar's fiancee, a status she vehemently denies, despite Edgar's overt courtship of her and multiple fairies calling her that. She obtained this status when she accepted a moonstone ring (previously owned by Gwendolen, the countess of one of the past Blue Knight Earls before Edgar), which effectively meant that she accepted his proposal. The moonstone ring apparently has some protective powers of its own; it prevented Lydia's soul from being fully trapped by the fairy world in volume 6 of the novel series, as well as enabled Lydia to break Kelpie's spell on her when he tried to bring her to the fairy world by magic (to prevent her from being hurt in the crossfire between Edgar and the Prince).
Recently, Lydia has truly accepted Edgar's proposal, and afterwards it is shown that Lydia has hence been accepted as part of the Blue Earl Knight's family (as Edgar's fiancee). When she takes what should have been a fatal blow from the Blue Earl Knight's sword, she emerges unscathed, proof that the sword has acknowledged her as the Knight's family. Following this revelation, she becomes able to form a shining bow from Gwendolen's moonstone ring, and is able to use it to repel Unseelie Court.