Lili Mathias practices as a doctor of magic law, partnering professionally with her brother, Maril Mathias. Together, they are renowned for their research in the field, investigating supernatural phenomena such as ghost-binding techniques during regeneration, direct scribing methods, and the fusion of magical artifacts—like combining a writ of passage to the endless abyss with tools such as Toru Muhyo's magic law book. A significant focus of their work involved studying aroropathy, a phenomenon characterized by dynamic energy interaction analogous to gravitational forces between celestial bodies, inspired by the ecological concept of allelopathy. They identified Toru Muhyo and Jiro Kusano as a potential aroropathic pair due to their energetic compatibility. Initially skeptical, Lili and Maril adopted the poetic term "The Red String of Fate" (or "The Golden String of Fate" in some regions) proposed by Page Klaus to describe this unique bond.

Lili possesses blonde hair and blue eyes. Her typical attire consists of red-rimmed glasses, a beige dress shirt fastened with a blue ribbon, a brown skirt, white socks, and brown shoes. She exhibits an analytical mind and blunt honesty, speaking her thoughts openly without reservation—a trait shared with her brother. Her personal interests include research, information gathering, pen and paper, gossip, current happenings, and cake. Her talents encompass a hyperactive imagination, detailed analysis, and dedicated research. She dislikes spicy foods and finds her brother Maril's stubbornness challenging.