Celistia Ralgris is a prominent figure in the Royal Officer Academy of the New Kingdom of Atismata, where she is a third-year student. She is a young woman of the Ralgris family, one of the Four Great Nobles, and holds the position of leader of the Knight Squadron. Her reputation as the academy's strongest Drag-Knight precedes her, and she commands the formidable Divine Drag-Ride known as Lindworm. Physically, she is a tall, buxom young woman with long blonde hair often held in place by a black headband and blue eyes.

Publicly, Celistia is perceived as a proud, strict, and formidable knight dedicated to protecting others, which has earned her a reputation for being a man-hater. However, this persona belies a more complex and socially awkward personality. In truth, she does not truly despise men but rather lacks the experience and confidence to interact with them comfortably. This misunderstanding has led to her isolation, and she often feels lonely, occasionally talking to herself or to a teddy bear to fill the silence. Despite her aloof exterior, she harbors a deep desire for friendship and social connection.

Her motivations and role in the story are heavily tied to a tragic event in her past. As a child, she was a beloved student of Wade Roadbelt, the maternal grandfather of the protagonist, Lux Arcadia. When a young Celistia informed her master of corruption within the old Arcadia Empire, Wade bravely spoke out against it. This act led to his imprisonment, and he ultimately died because of his challenge to the emperor. Celistia has carried an immense burden of guilt ever since, believing she was directly responsible for her teacher's death and that she wronged Lux and his family. This guilt is the true source of her initial hostility towards Lux when he enrolls at the academy, as she tries to distance herself from him to atone for her past and also to protect him from potential harm.

Her relationship with Lux Arcadia is the central axis of her character development. She initially demands his expulsion, but a significant turn occurs when she meets Lux while he is cross-dressing under the alias "Luno." During this time, she confesses her hidden loneliness and her difficulties with men. After Lux rescues her and reveals his true identity, he forgives her for her perceived crime against his family, absolving her of the guilt she had carried for so long. This forgiveness relieves her of her emotional burden, and she subsequently falls in love with him. She becomes his aide when he joins the Seven Dragon Paladins, supporting him both as a loyal comrade and a romantic interest. By the end of the series, she marries Lux, becoming one of his wives.

As a Drag-Knight, Celistia is considered nearly without equal, with her combat prowess being second only to Lux himself. Her abilities are honed to a state of completeness, with no conspicuous weaknesses. She possesses not only natural talent but also immense physical strength, which she cultivated to overcome the old empire's ideology that men were superior to women. Her primary weapon is a lance, and her abilities include generating and nullifying energy blasts, creating forcefields, and manipulating electricity. Her signature techniques include Lightning Lance and Starlight Zero, a wide-area extermination weapon. Lindworm also grants her a Divine Raiment known as Divine Gate, which allows for instantaneous teleportation. A notable technique is Break Purge, where she discards her heavy armor to drastically increase her speed and destructive power at the cost of her defenses.