Katsumi Liqueur is the central character of the story and a powerful sorceress serving the Attacked Mystification Police, or AMP, in Tokyo. She is the only daughter of Gigelf Liqueur, a renowned grand mage, and Fuyuka Liqueur, a priestess who was formerly a television news reporter. Born in Honolulu, Hawaii, and raised there, she initially lived an ordinary life as an office worker. However, her life changed when she traveled to Japan to visit her ailing mother, an event that led to her first encounters with supernatural entities known as the Lucifer Hawks and eventually to her recruitment into the AMP.

Katsumi is characterized by a personality that balances a fundamental gentleness and caring nature with a harsh, headstrong, and sometimes reckless determination in battle. Upon first joining the AMP, she was withdrawn and found it difficult to trust others or let them into her life, partly due to her anxiety about the immense magical powers she was beginning to control. This sense of isolation was compounded by a devastating revelation: she carries the distant blood of the Lucifer Hawks, the very monsters she has sworn to fight. This heritage, which she sees as a curse rather than a blessing, becomes a profound source of internal conflict, as she constantly fears that her demonic ancestry might surface and cause her to harm her friends.

Her primary motivation is shaped by a deep sense of inherited responsibility and a desire for atonement. She carries a heavy burden from her father's failed experiment, Project Gaia, which inadvertently allowed the Lucifer Hawks to invade the human world. This legacy pushes her to fight, often wanting to confront dangers alone to shield her comrades from harm. Within the AMP, Katsumi is more than just a member; as one of the most powerful magic users on the team, her role is often that of their strongest asset in the battle against supernatural threats. Her growth is central to the narrative, as she evolves from a reluctant and isolated recruit into a determined officer who, despite immense personal tragedy, finds a reason to continue fighting.

The relationships she forms are pivotal to her development. Her closest friendship is with fellow AMP officer Kiddy Phenil; they start as distrustful of each other but become each other's most trusted ally. Her romantic relationship is with a standard police officer named Robert "Roy" DeVice. This relationship is a source of great joy but also deep anxiety, as she worries her dangerous life and her Lucifer Hawk blood will make him a target. The tragedy that defines her character arc occurs when Roy is murdered by the antagonist Ganossa Maximilian. Overwhelmed by grief, Katsumi's demonic blood fully awakens, causing a destructive eruption of power that destroys her apartment. Consumed by sorrow and vulnerability, she goes missing and is later corrupted and possessed by the evil, sentient sword Medium, turning her into an enemy of the AMP. Her redemption comes when Kiddy sacrifices herself to break Medium's hold, an act that frees Katsumi and allows her to eventually return to the team. In a final twist that gives her a renewed will to live, she discovers she is pregnant with Roy's child, offering a sense of hope and continuity after her tragic fall.

Katsumi's notable abilities are rooted in her powerful magical lineage. She is a highly capable sorceress who can perform Hermetic magic, often manifested as instant runes and powerful energy blasts. Her primary weapon is Grospoliner, also known as the Emperor of Swords, a sentient magical blade that once belonged to her father. This sword is not merely a weapon; it has a will of its own and can change form, often folding into a skull-like shape when not in use. As the story progresses, she unlocks even greater powers hidden within her body, eventually surpassing even her father's magical abilities.