Lydia Carlton is a seventeen year old fairy doctor who was born and raised in Scotland. She is the daughter of Professor Carlton and inherited from her mother the rare ability to see and communicate with fairies. Her green eyes are a trait associated with this gift. She acts as a mediator between the human world and the realm of the fae, a role often met with skepticism in the rational Victorian society she inhabits. Lydia is independent and kind, and she does not hesitate to help others, especially when it comes to resolving problems between humans and fairies. She loves her father dearly and will do anything for him. Her goal is to become a full fledged fairy doctor, and she frequently places herself in danger to aid those in need.

Due to a painful childhood experience in which a love letter she received from a boy was later revealed to be a dare, Lydia dislikes the idea of falling in love and seems almost afraid of it. She struggles with self esteem issues, often referring to her own hair as rust colored and comparing her appearance unfavorably to her mother, who was known for her beauty. Despite her competence, she can be insecure and works hard to prove her worth through her efforts.

Lydia’s life changes when she meets Edgar J. C. Ashenbert, the enigmatic man who claims to be the legendary Blue Knight Earl. After helping him retrieve the Sword of the Merrow, she becomes his consultant on fairy related matters. Their relationship is complex and marked by trust issues, as Lydia often doubts Edgar’s sincerity, believing his declarations of love are merely part of his playboy persona. Over time, she accepts a moonstone ring from him, which effectively makes her his fiancée, but she vehemently denies this status. The ring itself holds protective powers, shielding her from fairy world magic. Her relationship with Edgar is central to the story, as she navigates his dark past and the dangers surrounding his quest.

Lydia is also accompanied by a fairy companion named Nico, who takes the form of a cat but insists on being treated as a refined gentleman. Nico often gathers information from other fairies and advises Lydia in fairy affairs. Lydia’s interactions with various fairies, including the troublesome water spirit Kelpie, showcase her knowledge and negotiation skills.

Her notable abilities include the power to see fairies that are invisible to most humans, communicate with them, and understand fairy lore. This makes her an invaluable bridge between the two worlds. Throughout the series, she develops from a cautious, self doubting young woman into someone more confident in her role and her feelings, though she remains wary of romance and continues to wrestle with her own insecurities.