Roswaal L. Mathers serves as a teacher within the parallel world of Isekai Quartet, originating from the Re:Zero universe. His body has been possessed by the soul of his ancestor, Roswaal A. Mathers, since he was fourteen years old. This generational possession, spanning approximately 400 years, was initiated to achieve the long-term goal of resurrecting his mentor Echidna, killed centuries earlier. The original Roswaal A. Mathers, born Altair Mathers, possessed immense magical potential but suffered uncontrolled mana causing physical pain until Echidna saved him. Her death drove him to rely on a Tome of Wisdom to guide his resurrection plans, necessitating body-hopping through his descendants to extend his lifespan. Standing 186 cm tall, he presents an eccentric appearance marked by heterochromia (yellow left eye, blue right eye), indigo hair, and flamboyant clown-like makeup featuring lavender face powder, purple lipstick, and elaborate asymmetric eye shadow. His attire typically blends jester-inspired elements: a miniature top hat, multicolored purple garments, a cape, and red-and-black boots. Without makeup, his features appear notably paler and gentler. This distinctive look often leads others to misidentify him as a clown. His personality blends theatrical eccentricity with profound manipulation. He speaks in exaggerated patterns and displays whimsical humor, yet conceals a calculating, ruthless intellect focused solely on his objectives. For centuries, he orchestrated tragedies, including permitting the Witch Cult's attack on the Oni village that orphaned twins Ram and Rem, whom he later adopted. He supported Emilia's royal candidacy solely to manipulate events toward Echidna's revival, viewing Subaru Natsuki's time-looping ability as instrumental. After losing a pivotal bet to Subaru, he was compelled to swear an Oath Bound Curse Mark. This binds him to genuinely support Emilia's faction and prohibits harming Subaru's allies under penalty of soul destruction. Despite this enforced cooperation, he maintains his ultimate ambition and subtly influences events. In Isekai Quartet, he adapts his manipulative tendencies to the school environment. As a teacher, he enforces discipline through punishments like detention or bucketing, while permitting transgressions that serve his interests, such as overlooking Megumin's explosion magic in science class. He leverages his authority to mediate conflicts, like restraining Albedo from retaliating against Aqua, and organizes activities such as talent shows or field days. His relationships mirror their original dynamics: he accepts Valentine's chocolate from Ram, reminds Subaru not to "judge a book by its cover," and disciplines Kazuma Sato's group for theft attempts. He participates directly in events, joining the teachers' team against students during a cavalry game. His capabilities center on the Divine Protection of Sorcery, granting mastery over all six elemental magic types. His power rivals an entire army, featuring flight, pyrokinesis (Goa, Ul Goa, Al Goa), and weather manipulation (Altemillion). He also possesses martial prowess and a yellow left eye that nullifies enemy spells. These abilities persist in Isekai Quartet, where he demonstrates flight and tactical spell usage during school competitions. Key relationships include his dynamic with Ram, whom he mentors and who remains devoted despite his manipulations; his transactional alliance with Emilia; his adversarial-turned-cooperative bond with Subaru; and his dismissive interactions with characters like Kyouhukou, whom he declines tea with. Recent developments show internal conflict regarding his approach toward Subaru's growth and authority, though whether this indicates genuine change remains unresolved.