Leen is a fairy who is over six hundred years old, though she appears as a young woman with silver hair styled in twin tails and golden eyes. Her youthful look is typical of fairies, who maintain this appearance throughout their long lives. She originally served as the tribal chief of the fairy tribe in the Kingdom of Mismede. After meeting Touya Mochizuki, she later becomes the court magician of Brunhild Dukedom and eventually one of his wives, taking the position of the fifth queen of the duchy.

Leen is a highly accomplished mage, capable of wielding six of the seven primary magic elements, with only the darkness element beyond her reach. She also commands several non-elemental spells, including Program, Transfer, Protection, and Discovery. Her magical skill is recognized to the extent that she serves as the teacher of Charlotte, the royal court mage of Belfast. In public, she uses light magic to conceal her fairy wings. She is frequently accompanied by a teddy bear named Paula, which she controls using her null attribute magic.

Leen's motivations are tied to her role as both a scholar of magic and a loyal supporter of Touya. Her deep knowledge and experience make her a valuable advisor and ally. She is calm, composed, and often takes a pragmatic approach to problems, though she also shows affection and care for those close to her. Her relationship with Touya develops from initial cooperation into a marital bond, and she integrates into the Brunhild household alongside the other wives. Throughout the story, she continues to refine her magical abilities and contributes to the defense and governance of the duchy, while also exploring the vast library of the floating facility Babylon. Her long life and accumulated wisdom give her a unique perspective among the main cast, and she remains a steady presence in the group's adventures.