Claire Kagenou is the elder sister of Cid Kagenou, the protagonist of the story, and is two years older than him, making her seventeen years old. She is the daughter of the Kagenou baron family, a rural noble house renowned for producing knights who use magic to enhance their bodies in combat. Having trained from a young age to uphold this family tradition, Claire is a highly talented swordswoman with exceptional magic control and swordsmanship skills, a fact that has earned the attention of the Midgar Kingdom's First Princess, Iris Midgar, who seeks to recruit her into the Crimson Knights after her graduation.

On the surface, Claire treats her brother Cid as a hopeless and unreliable younger sibling who shows no growth or ambition, often subjecting him to strict training regimens and scolding him for his lackadaisical attitude. She can be forceful and even violent, but this behavior is a misguided expression of her deep-seated love and concern for his future. In truth, Claire is an aggressive and devoted "brother complex" who is intensely protective of Cid. When Cid is falsely accused of kidnapping Princess Alexia Midgar, Claire confronts the authorities and Princess Iris herself, even attempting to storm the detention facility to rescue him personally. Her devotion is such that she deeply dislikes Alexia initially, viewing the princess as someone who is merely playing with her brother's feelings, though her attitude softens when Alexia compares Claire to Cid, an observation she takes as the highest compliment.

Claire's abilities are considerable. Her magical power and natural talent were significantly enhanced when she was younger due to a secret cure administered by Cid after she began showing symptoms of the dreaded "Demon Possession," an affliction that she was completely healed from before it could fully manifest. While she is not consciously aware of this enhancement, she has continuously refined her magic control and swordsmanship by unconsciously mimicking Cid's own movements. Her refined and aggressive swordsmanship allowed her to win the Bushin Festival, the kingdom's most prestigious swordsmanship tournament, after the top contender was eliminated. This victory brought her significant fame and further solidified her status as a leading young knight.

Throughout the story, Claire is repeatedly drawn into major conflicts, often because of her connection to her brother. She accompanies Cid to the Lawless City during the Red Moon event, an excursion she plans for him to earn recognition. It is there that she battles the Vampire Queen Elisabeth and is forcibly entered into a "possession contract" with the spirit Aurora. This event leaves a unique contract seal on her right hand, which she hides with a bandage, creating a special link between her and the legendary demon Diablos. Her importance is also recognized by the clandestine organization Shadow Garden, which views her as a "special VIP" and the sister of their leader, Shadow, ensuring she receives secret protection and special treatment without her knowledge. However, this protection cannot prevent her from becoming a target in the cult's larger schemes. During a subsequent terrorist attack on the academy, Claire is captured and used as a catalyst for a ritual to resurrect Diablos, resulting in a new magic circle being engraved on her right hand and leaving her in a deep, seemingly endless comatose state from which she has yet to awaken.