Claude Sirius is a central character in the anime The Classroom of a Black Cat and a Witch, known originally as a prodigious sorcerer who became the youngest instructor in the history of the prestigious Royal Diana Academy of Magic at only seventeen years old. Before the events of the story, his life took a dramatic turn when he was afflicted by a curse from the terrorist cult known as The Black Serpents during their assault on the academy, which transformed his body into that of a black cat and caused him to disappear from the magical world. In this feline state, he is often called by the nickname Grim.

As a personality, Claude presents a complex mixture of arrogance, sharp-tongued sarcasm, and a pragmatic, self-serving attitude that initially masks his deeper nature. He often hides a caring and fiercely protective core beneath this abrasive exterior, displaying a reluctance to show vulnerability or emotion. Haunted by guilt over his past, particularly concerning events involving students and allies during the academy's siege, he tends to refuse to disclose the details of his history, which has led to misunderstandings with former peers and students. Despite his tendency toward impatience and bluntness, especially when dealing with the early struggles of his apprentice, he fits the archetype of a "jerk with a heart of gold," demonstrating selfless bravery by confronting dangerous enemies to protect others.

Claude's primary motivation is to break the curse that traps him in the body of a black cat, which severely limits his access to his magical powers. This goal leads him to form a secret pact with Spica Virgo, an aspiring witch who lacks magical ability but possesses the unique latent power of Rebirth magic necessary to reverse his condition. In exchange for her help in restoring his human form, he pledges to mentor her in magic, though the curse can only be temporarily lifted each morning through an embarrassing ritual that requires Spica to kiss a specific part of his cat body. His motivation evolves throughout the narrative as he grows from a reluctant teacher focused on his own curse into a deeply invested guardian dedicated to training Spica and protecting his students.

Within the story's role, Claude serves as the secret master and mentor to Spica, creating a hidden teacher-student relationship that forms the foundation of the plot. He becomes the instructor in charge of Pandora's Box, a class of eclectic students at the academy, where he transitions from a suspicious and missing figure into a returning professor who insists his presence is for the sake of his pupils. His return to the academy is met with mixed reactions, as his sudden disappearance two years prior had led many to believe he abandoned his students during the attack.

Key relationships define much of Claude's character arc. His bond with Spica Virgo is the most central, growing from a transactional arrangement into a deep mutual trust and a guardian-like connection, though he remains oblivious to her romantic feelings toward him. He also has significant relationships with other students, including Castor Gemini and Astraea Libra, whom he reassures upon his return, and a fractured history with Pollux Gemini that contributes to his guilt-ridden personality. His past with a former ally named Arc, who believes he betrayed her, creates tension and underscores themes of sacrifice and eventual redemption.

Character development for Claude involves his gradual evolution as a mentor and a person. Initially self-serving and only interested in breaking his curse, he shifts toward becoming a loyal and protective teacher who confronts threats without hesitation to defend his students. His past actions, deliberately drawing danger away from students during the cult attack, reveal a selfless core that becomes more apparent as the story progresses and misunderstandings about his disappearance are cleared. He transforms from a cursed, isolated figure into a respected if complicated instructor who learns to trust and invest in his pupils.

Notable abilities define Claude as an exceptionally powerful sorcerer, ranking among the mightiest in the setting even in his diminished state. He commands a type of magic known as Sagittarius magic, specializing in wind manipulation and ice conjuration. Among his demonstrated techniques are Whirlwind, which creates a strong gust of wind to shield himself and others; Icing Arrow, which forms a large bow and arrow made of ice that rapidly freezes its impact surface; and Dragon Bow: Storm Javelin, a Grand Magic technique that absorbs turbulent wind currents and condenses them into a powerful shot capable of dispersing cloud cover. The Curse of Reincarnation, however, has essentially vanished his original human body and limited his current feline form to only restricted access to his magic, which he can channel through special earrings. While in his cat form, only Spica can understand his speech, with other characters hearing only ordinary meows. His human form can be temporarily restored through Spica's Rebirth magic, but the duration depends entirely on her remaining mana reserves, and he reverts to his cat form if she runs out of energy or if he uses too much of his own magic. A permanent cure requires Spica to attain magical power equal to his own, fueling his dedication to her rigorous training. When fully restored to human form, his power proves formidable enough to overcome major adversaries like the cult members who cursed him.