Clarus Tonitrus is a 15-year-old operative of Fatima, the Vatican's secret Anti-Creed service combating demonic Nights. Trained rigorously from early childhood within the organization, this background forged her intense combat focus and strict adherence to protocol. Her operational history is defined by partnership; initially paired with Flors Oriens for half a year, she endured a defining trauma when Flors advocated for peace with the Nights and subsequently fell into a coma after an attack by Creed Killer. Clarus internalized this loss as a personal failure, a "sin" of opening her heart too much. This event triggered profound emotional withdrawal, extinguishing her capacity to smile and fueling an exclusive focus on vengeance against the Nights. She refused new partnerships for three months following the incident, devoting herself entirely to eradicating Nights and awaiting a confrontation with Creed Killer. Her constant vigil at Flors's bedside and visible relief upon Flors's eventual awakening reveal the enduring depth of this bond.

Her combat prowess, classified as Building Level in attack potency, durability, and striking strength, centers on wielding a staff and armor. Key powers include Superhuman Physical Characteristics, Teleportation via travel through higher dimensions (Gate of Caelum), Forcefield Creation, Banishment/BFR by sending targets to higher dimensions, Electricity Manipulation via her staff, Dimensional Travel, Portal Creation, Existence Erasure (capable of completely erasing Nights), and Regeneration (Low). She possesses resistance to Time Manipulation, Spatial Manipulation, Higher Dimensional Manipulation, and Existence Erasure, attributed to protection from her Higher-Dimensional Being which renders lower-dimensional hax ineffective. These capabilities are amplified in her God Clad Armor states. Her combat style is direct and powerful, capable of destroying Nights larger than buildings.

With blonde hair and blue eyes, her appearance garners significant attention in civilian settings, noted as an Informed Attractiveness that often draws fascinated stares, sometimes requiring her to physically remove obstacles. Socially, her upbringing within Fatima and the trauma of losing Flors manifest as pronounced stoicism and difficulty interacting outside her organizational framework. She struggles to socialize with non-Anti-Creed individuals and actively avoids forming new connections to prevent further loss, defining her initial interactions.

Her dynamic with Matoi Sumeragi begins with skepticism and surveillance, wary of Matoi's newfound powers. Their relationship evolves through shared experiences: Clarus confirmed Matoi's divine nature by partially disrobing her after purging a demon targeting her. Moments of vulnerability surfaced, such as embarrassment when Matoi complimented her smile, causing Clarus to turn away, and physical reactions like feeling overheated when Matoi clung to her arm during a test of courage. Significant intimacy milestones include sharing Matoi's bed overnight and a pivotal moment where Clarus held Matoi's hand and winked before journeying to the highest dimension. While operating alongside Matoi and Yuma Kusanagi, Clarus maintains a watchful, initially distant role.

Her development involves a gradual, reluctant thawing of her emotional barriers through her forced partnership with Matoi, contrasting sharply with her previous partnership with Flors. The culmination involves confronting Creed Killer and witnessing Flors's awakening, allowing some release from her guilt and self-imposed emotional isolation. Her relief at Flors's recovery indicates a potential for healing, though her core dedication to combating Nights remains.