Nagi Kirima is the daughter of the famous writer Seiichi Kirima, an eccentric and enigmatic author whose works on criminal psychology and philosophy shaped much of her worldview. Her mother passed away, and she lives on her father's inheritance. She spent a period of time in a hospital as a child because her body was undergoing an uncontrolled evolutionary process that threatened her life. While hospitalized, she met Shinpei Kuroda, an agent of the Towa organization, who betrayed his employers to administer a stolen experimental drug that saved her. This event, combined with her father's later death, set her on a path of relentless self-discipline and a self-appointed mission to protect the world from supernatural and existential threats. She is the older non-blood-related sibling of Masaki Taniguchi, with whom she shares a close and protective bond, though she keeps her distance emotionally to avoid drawing him into danger.

Known and feared around her school as the Fire Witch, Nagi is a tall, athletic young woman with short, spiky dark hair and intense eyes. She speaks in a blunt, boyish manner and uses masculine first-person pronouns in the original Japanese text, though she adopts a more standard feminine form in the anime. Her school treats her as a troublemaker due to her rebellious attitude and habit of skipping class to investigate paranormal occurrences, yet her grades remain excellent. She is stoic, intelligent, and fiercely independent, guided by a personal code of justice that sets her apart from her peers. She distances herself from others because she believes she is too dangerous for normal people to associate with, and she is quick to act, often stepping into danger without hesitation. She has a messianic complex and seeks to save the world from whatever may threaten it, seeing herself as a protector who must act because others cannot or will not.

Her motivation stems from the trauma of her childhood hospitalization, the influence of her father's philosophical writings, and her awareness of the hidden horrors that permeate the world. She is driven by a deep sense of responsibility and a desire to prevent others from suffering as she did. She investigates any unusual happenings in the city, following leads on disappearances, psychic phenomena, and synthetic humans. In Boogiepop Phantom, she is particularly focused on the electromagnetic interference that followed a pillar of light in the night sky, seeking to understand and stop the strange events it causes.

Nagi has met and conversed with Boogiepop several times and is one of the few people who knows the entity's identity. Their relationship is one of mutual recognition as parallel protectors, though their methods differ. She also has a good rapport with Kentaro Habara, a teacher who shares her investigative instincts. Among her peers, she maintains a cautious connection with Naoko Kamikishiro and Kazuko Suema, who recognize her brilliance and understand her inner struggles. Her father, Seiichi Kirima, remains a key influence even after his death, and her last conversation with him strongly shaped her ideals and her perception of reality.

Throughout her arc, Nagi undergoes significant development as she confronts her own inner nature. Unbeknownst to her at first, she is the vessel of Walpurgis, the Witch of Fire, a powerful and destructive force destined to clash with her eternal counterpart, the Witch of Ice. This presence nearly destroyed her during her childhood until it was suppressed through Shinpei Kuroda's intervention. Her journey involves coming to terms with this power, learning to control it, and reclaiming agency in the face of a destiny she never asked for. She seeks not domination but balance, and her struggle is one of self-discovery, restraint, and philosophical resistance against forces that seek to define or erase individual will.

Nagi is extremely gifted both physically and intellectually. She has trained in combat from a young age, using martial arts as a way to channel and control the latent energy within her. Her fighting skills allow her to take down criminals and supernatural threats alike. She possesses sharp analytical and deductive abilities, enabling her to investigate complex mysteries and uncover hidden truths. Additionally, she carries the dormant power of Walpurgis, which grants her immense latent strength, though she must consciously restrain it to avoid losing control. Her given name Nagi means calm sea after a storm, an ironic contrast to the fiery power and turmoil she carries within. Her arc touches on themes of repression, identity, fate, and the burden of power, making her one of the most complex characters in the series.