Lydia Carlton, the daughter of Professor Carlton, is a 17 year old fairy doctor. She has the rare power to see fairies, an ability she inherited from her mother. Lydia also possesses green eyes, a trait of those with the gift to see fairies. She is independent and kind, Lydia doesn't hesitate to help others. She loves her father dearly and will do anything for him. She ends up working for Edgar as his consultant on fairy-related matters after helping him retrieve the Merrow's Sword (key to him being recognized as the Blue Knight Earl by the Queen of England). 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 cast a spell on Kelpie so that he would not come near her, as his aggressive advances were troublesome to her. She doesn't believe Edgar when he reveals his love for her, and only believes he is just being his usual playboy self.

Her goal is to become a full-fledged fairy doctor, and 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 apparently a woman of considerable beauty; everyone who knew her mother before she passed away says that they don't look anything alike. Once, in an outburst, she confesses to Paul Ferman that she thinks that she isn't cute at all, and that it's part of the reason why she tries so hard to help in other ways (by working hard).

Lydia currently is considered Edgar's fiancee, a status she vehemently denies, despite Edgar's 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 enabling 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).