Celenike Icecolle Yggdmillennia is a magus belonging to the Yggdmillennia clan, a group of mages who oppose the Mage's Association in the Great Holy Grail War. She serves as a Master of the Black faction, summoning the Servant Rider, Astolfo, into the conflict. Physically, she is described as having a clever and beautiful appearance, standing 168 cm tall and born on December 11th.

Her background is rooted in the Icecolle family, a lineage known for its practice of dark magecraft and curse-based magic, or witchcraft. The family's rituals required its practitioners to maintain a cold, rational demeanor when dealing with sacrifices, an attitude meant to mirror the solemnity of preparing meat. However, the cumulative weight of their sacrifices caused the curses binding the family to grow too strong, leading to their decline. Celenike herself was a rare prodigy born into this decaying family. She was doted upon by the elder female magi of the Icecolle family, who taught her the intricacies of black magic. To perfect her craft, she was required to suppress any feelings of pleasure derived from inflicting pain, fostering a chilling rationality. The strict suppression of emotion during rituals had the adverse effect of leaving her with no self-control in her personal life, particularly regarding her sadistic impulses.

Personality-wise, Celenike is defined by cruelty, sadism, and a prioritization of her own desires above all else, including the goals of her clan. She is known to have run a business cursing others to death, using her magical talents to kill for profit. Her preferred targets are the weak, as she harbors a deep-seated hatred for the strong and any form of betrayal. While she possesses a calm and composed demeanor as a magus, this facade masks a deeply distorted and violent nature. Her body is said to perpetually carry the scent of blood from her grisly rituals, which she attempts to mask with perfume. Despite her ancient magical lineage, she is also shown to be adaptable, utilizing modern technology such as computers to deliver her curses through electronic networks.

Celenike's primary motivation throughout the story is not the success of her faction or obtaining the Holy Grail, but rather the fulfillment of her own twisted obsessions. This becomes most apparent with the summoning of her Servant, Astolfo. She became captivated by his form and developed an intensely possessive and abusive fixation on him. She viewed the heroic spirit not as a partner or a legendary figure, but as a pawn and an object for her amusement and gratification. Her private treatment of Astolfo involved acts of bondage, torture, and sexual abuse, which she considered her right as his Master. Her interest in the Grail War was solely as a means to an end, a period during which she could indulge in her desires with Astolfo without consequence.

Her role in the narrative is primarily that of an antagonist within the antagonistic Yggdmillennia faction itself. She begins as a member of the Black camp, but her loyalty to the clan is purely pragmatic. She does not share Darnic Prestone Yggdmillennia's ambition for the clan's independence; instead, she sees the war as a convenient opportunity for her own pleasures. Her story takes a decisive turn when Astolfo begins to show compassion and loyalty towards a homunculus named Sieg, who escapes from the Yggdmillennia laboratory. This triggers a violent jealousy in Celenike. Enraged that her property is showing affection for another, she grows increasingly unstable and abandons her duties as a Master to focus on punishing her Servant and destroying Sieg.

Key relationships define her downfall. Her relationship with her Servant, Astolfo, is a toxic dynamic of abuse and possession. She constantly punishes him for perceived disobedience, contrasting sharply with his own chivalrous and kind nature. Her hatred for Sieg is born of intense jealousy, as she recognizes that Astolfo has formed a genuine bond with the homunculus, something he never shared with her. This jealousy drives her to a state of near-madness. Her relationship with her own clan is transactional; the other members, aware of her talents, turned a blind eye to her sadistic hobbies, a decision that would ultimately contribute to her demise. Fellow Masters like Fiore and Caules Forvedge Yggdmillennia hold her in contempt, and her death elicits no sympathy from them.

Celenike undergoes little development in the traditional sense, instead devolving further into her own mania. As the war progresses, her rationality erodes completely, replaced by a singular desire to break Astolfo's spirit. In the climax of her character arc, she confronts Sieg and Astolfo on the battlefield. Using her Command Spells, she orders Astolfo to kill Sieg in a gruesome manner, intending to revel in her Servant's pain and despair. As she prepares to issue another Command Spell to overcome Astolfo's resistance, she is killed from behind by Saber of Red, Mordred, who dismissively decapitates her as an obstacle. Her final moments are not of fear but of twisted euphoria, as she was lost in the fantasy of breaking Astolfo, unaware of her own end.

In terms of notable abilities, Celenike is a highly skilled practitioner of black magic. Her specialty is curses that cause death, and she possesses numerous magical items and tools for this purpose, such as cursed nails that inflict severe pain. She is meticulous and relentless in her pursuits, capable of tracking a target down and eliminating them through any means necessary. Her proficiency is such that she can even curse individuals remotely via the internet, showcasing an innovative combination of magecraft and modern technology. Despite her magical competence, her psychological instability and inability to form a proper contract with her Servant ultimately prove to be her fatal weaknesses.