Clarus Tonitrus is a fifteen-year-old operative of Fatima, a secret exorcism branch of the Vatican dedicated to eliminating extradimensional entities known as Nights. Trained from early childhood within this organization, she possesses a keen intelligence and extensive knowledge of higher-dimensional beings and tactical combat. Her appearance is marked by blonde hair and blue eyes, and her civilian attire typically consists of light purple clothing.

Clarus maintains a stoic, serious, and initially冷酷 (rèikoku) or cold demeanor, showing no mercy to her enemies. This personality is a direct result of a traumatic past event involving her previous partner, Flors Oriens. After collaborating with Flors for half a year, an advocate for peace with the Nights, Flors was incapacitated and fell into a coma after an attack by a powerful Night named Creed Killer. Clarus internalized this loss as a personal failure, believing that her openness towards Flors was a fatal weakness. This scarring event drove her to pursue the annihilation of all Nights without mercy, caused her to shut down emotionally, and erased her ability to smile. She subsequently refused further partnerships, dedicating herself entirely to her mission of exterminating Nights and waiting for a confrontation with Creed Killer.

For most of the series, Clarus relies on a simulated form of Divine Union, the transformative fusion with a higher-dimensional god that grants exorcists their powers. Her god is synthesized through a special piece of equipment housed in a simple suitcase, which dons her in a primarily black suit of armor and allows her to use a variety of weapons. In this state, she wields a staff and can generate powerful lightning attacks, create forcefields, teleport, and banish foes to higher dimensions. However, her self-imposed emotional barriers initially limit her power, preventing her from transcending beyond the 12th dimension, as exceeding her god's perceived limits could lead to her own existence being erased. It is only in the final episode, when facing a dire crisis, that she confronts the god within herself and unlocks her true Divine Union power, at which point her costume transforms into an all-white ensemble.

Clarus's role in the story is that of an experienced and powerful ally to the protagonist, Matoi Sumeragi. She is introduced as a knowledgeable foil to the reluctant and newly empowered Matoi, initially observing her with skepticism. Their relationship is the primary catalyst for Clarus's emotional development. Through forced partnership and shared experiences, she gradually forges a bond with Matoi. In one notable early interaction, Clarus purges a demon targeting Matoi, and in an effort to confirm Matoi's divine nature, she accidentally damages her clothing, an act that leads to a moment of embarrassment for Clarus when Matoi compliments her. As their connection deepens, Clarus experiences moments of vulnerability, such as sharing a bed with Matoi and clasping her hand before dimensional travel. Matoi's persistent warmth and openness slowly cause Clarus's emotional barriers to thaw, allowing her to confront the past trauma that has defined her. Her arc reaches a turning point with the eventual awakening of Flors from her coma, which provides Clarus with significant relief from her long-held guilt and anguish, suggesting a path toward healing and an ability to smile once more.