Tia Agatha serves as the primary guardian and manager for Kaz Kaan, having raised him since childhood after his parents vanished. She oversees his demon-slaying duties, a requirement of their family's privileged Magistocrat status within Neo Yokio, earned through generations protecting the city. Agatha strictly enforces these obligations, frequently compelling Kaz to undertake exorcism missions against his will, driven by pragmatism and an intense preoccupation with safeguarding the family's social standing and finances. To her, the threat of social humiliation or reputational damage outweighs Kaz's personal reservations.

She possesses distinctive pink hair and purple eyes, often wearing sunglasses during assignments and favoring practical trench coats over formal attire. Her familial relationships reveal complexity, particularly when her fraternal twin sister, Tia Angelique, visits during Christmas. The sisters share a contentious dynamic rooted in conflicting values: Agatha prioritizes wealth and status, while Angelique embraces simpler ideals. This conflict escalates when Angelique references their concealed demonic heritage—a revelation Agatha fears could destroy their reputation and safety. Agatha expels Angelique, demonstrating her prioritization of status over familial bonds.

The discovery of their demon lineage marks a pivotal moment. When a possessed character reveals the truth to Kaz, Agatha confirms their ancestry and orders Kaz to eliminate the threat to protect their secret. This directive underscores her unwavering prioritization of societal preservation and family reputation above all else, including moral ambiguity. This revelation also contextualizes her lifelong emphasis on controlling Kaz's actions and upholding the Magistocrat facade, as exposure risks both their elite status and physical safety. Her character development remains anchored in themes of concealment, control, and the burdens of legacy.