Mycroft Swallow is a student at the same school as William Twining and serves as a fellow prefect at Stratford. He is a young man with dark blue hair and dark blue eyes that often appear purple or lilac. Mycroft is highly intelligent, having been educated in fields such as philosophy, literature, and economics from a young age, which gives him a broad base of knowledge. He is characterized by a calm, patient, and collected demeanor, rarely showing strong emotions even in difficult circumstances. He is also reserved and humble, preferring to stay in the background rather than draw attention to himself. His unwavering loyalty and discretion make him a reliable confidant.

Mycroft's primary motivation is his deep devotion to William Twining. He consistently places William's needs above his own and provides counsel and support during the various supernatural and political challenges William faces. His calm presence often serves as a source of comfort for William, and he is willing to risk his own safety to protect his friend. In the story, Mycroft occupies the role of a close friend and ally, distinct from the more flamboyant demon characters. He is noted as being one of the few human students who regularly interacts with William, alongside Isaac.

While Mycroft does not possess supernatural abilities, his notable talents lie in his sharp intellect, strategic thinking, and analytical nature. He approaches problems logically and can manipulate situations to achieve his goals. His personality has been compared to the INTJ type in the Myers-Briggs system and the Enneagram Type Five, reflecting his investigative nature and preference for observation over social participation. His character remains consistent throughout the series, with his steadfast loyalty and intelligence being his defining traits, though he shows subtle growth in his willingness to engage with the extraordinary events surrounding William.