Nagisa Akatsuki is Kojou Akatsuki's younger sister, attending Saikai Academy's junior high division in class 3-C alongside Yukina Himeragi and Kanon Kanase. She sports black hair, typically worn in a high ponytail secured by a mint-blue ribbon with bangs framing her forehead, and reddish-brown eyes. Her Saikai Academy uniform is her usual attire, and her hair is noted to be longer than her signature ponytail suggests. Four years prior, Nagisa suffered a critical injury during the excavation of Avrora Florestina, inflicted by the Black Death Emperor Front. This trauma was masked by a fabricated memory of a demon-caused train accident in Rome. Treated on Itogami Island in a vegetative state, she became unknowingly possessed by "Root." Three and a half years later, Root awakened, using her body as the Fourth Primogenitor to consume other Kaleid Bloods and trigger a mass vampire-turning event on the island. Kojou and Avrora ultimately destroyed Root. Following this, Nagisa transferred Avrora Florestina's consciousness into her own body, resulting in the loss of her spiritual medium powers and all memories related to Avrora. Her mother, Mimori Akatsuki, became her personal physician. Nagisa displays a cheerful, playful, and chatty personality, especially around those she likes such as Kojou or Yukina. She can struggle to read social cues, exemplified by her excited reaction to La Folia Rihavein kissing Kojou without noticing the jealousy of others present. She dislikes Sayaka Kirasaka due to a past hostile encounter where Sayaka attacked Kojou and restrained Nagisa. A notable quirk is her habit of addressing family members by their given names. She shares a close bond with her brother Kojou. Her strong friendships include Yukina Himeragi, Kanon Kanase, and Yuuma Tokoyogi, whom she first met when Kojou and Yuuma retrieved her hat from a cliff edge. Skilled at cooking, Nagisa regularly prepares meals for her family. An avid cat lover, she assisted a classmate in caring for stray cats. The spin-off manga depicts her surreal daily life with Yukina and Kanon as close friends. Originally, Nagisa possessed abilities as a spirit medium trained by her grandmother, inheriting hyper-adapter blood from her mother. This granted her psychometry, enabling her to locate ancient ruins and decipher epigraphs—a skill sought by academic institutions. This power was lost due to her trauma. Presently, Avrora Florestina's consciousness resides within her. As the 12th Kaleid Blood, this allows the summoning of the 12th Beast Vassal, Alrescha-Glacies, capable of lowering temperatures to absolute zero. Nagisa remains unaware of Avrora's presence and cannot consciously access this power. Throughout the series, Nagisa confronts the true root cause of her demon-related trauma—the Root possession—rather than the fabricated train accident. She later plays a pivotal role by employing the ritual "Teokratia." This involved implanting her own memories of daily life on Itogami Island into the girls serving as Beast Vassal Warhead icons. These fused memories, carrying the imprint of Avrora’s soul, provided the icons with nostalgic and absorbable experiences. Accessing Nagisa’s feelings empowered them to seize futures where they coexisted with the tamed Beast Vassals.