Nagisa Akatsuki is the younger sister of Kojou Akatsuki and a middle school student attending Saikai Academy, where she shares a classroom with Yukina Himeragi and Kanon Kanase. She is an outgoing and talkative person with a warm disposition, often described as gregarious and extraverted, and she quickly forms close friendships with those around her, including her childhood friend Yuuma Tokoyogi. A skilled cook, she is known for her domestic abilities and her caring nature toward her brother and friends.

Her background is marked by a traumatic incident four years before the main story. She was involved in a train accident caused by demons, which left her with a lasting fear of demonic beings. In reality, this event was a false memory implanted to cover her involvement in the excavation of the twelfth Kaleid Blood, Avrora Florestina. Nagisa is an extremely rare hybrid: she inherited the qualities of a spirit maiden, or miko, from her paternal grandmother and the powers of a Hyper Adapter from her mother, Mimori Akatsuki. She was trained as a spirit medium and developed psychometric abilities that allowed her to read the history of objects and locations, abilities for which she was sought by universities and institutions. However, she lost these powers due to the psychological trauma she endured.

Nagisa's role in the story deepens significantly when she becomes a vessel for powerful forces. After being attacked by the Black Death Emperor Faction, she fell into a vegetative state and was brought to Itogami Island for special treatment. Unbeknownst to those around her, she had been possessed by an entity known as Root, which awakened as the fourth primogenitor within her body. Root caused widespread chaos, creating a mass vampire-turning incident and feeding on human memories. During the confrontation, Kojou and Avrora managed to destroy Root, but just before Avrora perished, Nagisa transferred Avrora's consciousness into herself. As a result, Avrora's personality now resides within Nagisa, granting her the ability to summon the twelfth familiar, Alrescha-Glacies, which can decrease temperatures to near absolute zero. This event also caused Nagisa to lose all memories related to Avrora, and her mother became her personal physician to manage her condition.

Throughout the series, Nagisa serves as a grounding presence for her brother and friends, offering emotional support and a sense of normalcy despite her extraordinary circumstances. Her development involves overcoming her trauma and coming to terms with the power and consciousness she now carries within her. Her key relationships include her protective bond with Kojou, her close friendship with Yukina, and her enduring connection to Yuuma. Nagisa remains a pivotal supporting character whose personal journey entwines with the larger conflicts surrounding the primogenitors and the supernatural world.