Ranmaru Ai holds the title of Fairy of Illumination within the prestigious Lux Clan of the fairy world. He appears as a slim young man with fair skin, messy mauve hair, and dark orange eyes, always wearing glasses in his human form. His typical attire is a white school blazer with blue lining, matching white trousers, and a ring on his right middle finger.

Honest and kind, Ranmaru dedicates himself to healing afflicted human hearts by collecting "Attachments." He frequently mediates conflicts among companions, displaying high perceptiveness that clashes with limited social skills, leading to blunt questions. While typically scatterbrained, he becomes intensely focused and combat-proficient during missions. His human form requires glasses for clear vision, though they are lost during transformations.

Ranmaru suffers from amnesia, obscuring a past hinted to be far older than his appearance suggests. Fragmented memories reveal he survived a rebellion that destroyed the previous fairy kingdom. This rebellion connects to his shattered left wing, an injury sustained during a suicide attempt after he betrayed the fairy realm. His forgotten history involves unrequited feelings for Sirius, a fallen fairy antagonist; Ranmaru remains unaware these feelings contributed to Sirius's betrayal, which caused the kingdom's destruction and triggered his own self-destructive act.

In the present, posing as a high school student and working at Bar F under the Fairy Queen's orders, Ranmaru wields a fencing sabre in combat and utilizes light-based magic. His fairy form features a purple color motif, a backless outfit, and damaged wings. Ongoing tension stems from his forgotten past with Sirius. His missions require entering clients' psychological worlds, defeating manifestations of corruption, and purifying Attachments for the queen's undisclosed purposes.