Nana Takashiro is the younger sister of Natsume and Shuugo Takashiro, attending Kayou Private Academy in Ueno. She stands 143 cm tall, weighs 34 kg, and has measurements of 67-47-70 cm. Her appearance features short brown hair parted to the side, violet eyes, glasses, a hairpin, a scarf, her school uniform, and thigh-high stockings.

Her personality is blunt, talkative, and strange, often using "boku" as her first-person pronoun. She harbors a brother complex, particularly toward Shuugo. Possessing genius-level intelligence, encyclopedic knowledge, and a strong memory, she habitually makes bizarre statements or engages in unusual behaviors that concern her brother. Nana openly identifies as a pervert and demonstrates a strong fascination with the bizarre serial murders occurring in Ueno. Despite Shuugo's attempts to stop her, she initiates her own investigation into these killings. Notable quotes include "There is nothing in this world that I do not know" and "There's no better way to learn than by doing. It looks like I'm going to have to cross the Rubicon here."

Beyond her student role, she actively pursues her investigation, interacting with Shuugo's acquaintances like her classmate Ayasaki Takako. Her inquiries become entangled with the central mystery involving the Kozuki family and the Senri cult. In the fandisc after story, she deliberately leaves a knife accessible to Kozuki Yura, enabling Yura's escape from a hospital and subsequent killing spree. This action indirectly leads to Yura's death when Touji Aoki intervenes.

Her name "Nana" (七七) means "seventy-seven," and "Takashiro" (高城) combines "tall/high" (高) with "castle" (城).