Ciel, also known by her real first name Elesia, is a character who leads a double life. She is first introduced as a senior student at the same school as the protagonist, Shiki Tohno. In this public role, she is the president and only member of the tea ceremony club, presenting the demeanor of a calm, friendly, and peaceful person. She quickly becomes good friends with Shiki and his close companions, Arihiko Inui and Satsuki Yumizuka. However, this ordinary student persona is a cover for her true identity. Ciel is a highly skilled member of the Burial Agency, a secret and feared organization within the Catholic Church. The Agency is composed of assassins known for their ruthless efficiency, willing to use forbidden magic and creatures to eliminate heretics. Her mission in Tsukihime is to hunt down and destroy the vampire known as Roa.

Ciel's background is tragic and complex, directly tying her to the very vampire she hunts. Born in France to a French father and an Oriental mother, she was a normal girl with unusually high magical potential. This rare characteristic caused her to be chosen as the seventeenth reincarnation of the immortal vampire Roa. After her sixteenth incarnation was killed, Roa’s soul entered the young Elesia. As Roa manifested within her around her sixteenth birthday, he took control, forcing her to commit atrocities, including massacring the people of her village. This event only ended when Arcueid Brunestud, the vampire who hunts Roa across his reincarnations, arrived and killed her body. Unlike Roa's previous hosts, Elesia's body revived after his soul had already moved into its next host, a young man named SHIKI Tohno. The Church captured her, and despite being killed repeatedly over a month in an attempt to destroy her, she continued to revive due to a paradoxical immortality tied to Roa's continued existence. Recognizing her unique nature and abilities, the Church's Burial Agency recruited her instead of executing her.

Personality-wise, Ciel displays two distinct but genuine facets. The gentle, helpful, and reliable senpai seen at school is one aspect of her personality. She enjoys relaxing in the tea ceremony room, drinking tea, helping others, and even tricking the school administration into giving her a budget for candy. Her other personality is that of a cold, serious, and calculating executor who is willing to do whatever it takes to complete her mission. This side is driven by a deep personal vendetta against Roa, as destroying him is the only way she can end her cursed immortality and the eternal existence she despises. Despite her attempts to remain detached, her interactions with Shiki make it difficult for her to maintain her cold facade. She also has notable quirks, including an obsession with curry, which she discovered on a mission to India, and a fetish for designing her own combat outfits.

Ciel's role in the story is multi-faceted. Initially, she is undercover at Shiki's school primarily to investigate whether he might be the current incarnation of Roa. As the narrative progresses, she becomes a key ally, particularly in the fight against Roa, who resides within Shiki's adopted brother. Her relationship with Shiki is central to her character development. What begins as a mission of surveillance develops into genuine affection and love, which creates a profound internal conflict between her duty as a cold-hearted hunter and her personal feelings. This conflict comes to a head in her own route of the story, where she confesses that her initial purpose was a lie but ultimately cannot bring herself to kill him. Her relationship with Arcueid is more antagonistic, as Ciel is a member of the Church and Arcueid is a powerful vampire, leading to a mutual tolerance rather than a partnership against their common enemy, Roa.

Ciel possesses a formidable array of abilities that make her a powerful combatant. Her primary weapons are Black Keys, which are conceptual weapons that appear as sword hilts that project a blade of light. They are thrown as projectiles and can immobilize a target by pinning their shadow. Her trump card is the Seventh Holy Scripture, a massive conceptual weapon that resembles a pile bunker or cannon. Created from a unicorn's horn and infused with a soul, it is designed to destroy both the body and soul, negating the regeneration and immortality of vampires. Due to having once been Roa’s vessel, she has access to his vast accumulation of magical knowledge spanning a millennium, though she is reluctant to use it. She also possesses the Mystic Eyes of Whisper, which allow her to hypnotize and implant false memories in people, a power she uses to seamlessly integrate into the school. Her most notable ability is her paradoxical immortality; as long as Roa exists, the World will continue to reverse her death and regenerate her from any injury, making her nearly unkillable until her primary goal is achieved.