Nao Saitō, initially known as Yakumo Saitō's younger cousin, is later revealed as his paternal half-sibling, sharing the same father. Born on April 15th, she has been deaf since birth but compensates with heightened sensitivity to others' emotions, perceiving their feelings clearly. She resides early on with guardian Isshin Saitou—a Buddhist priest serving as her father figure—at his temple. Her biological mother, Akemi Takagishi (Yakumo’s teacher), held unrequited feelings for Isshin. Nao had a twin brother, Yuuta Takagishi, born with distinctive red eyes; he died shortly after birth when an unconscious Akemi accidentally dropped him.

Shy around strangers in childhood, Nao formed close bonds with Isshin and Yakumo. Following Isshin’s death in volume 6 of the series, she is adopted by Kazutoshi Gotou and his wife Atsuko, temporarily relocating to Isshin’s former temple.

Nao exhibits a kind-hearted, perceptive, and gentle personality, naturally drawing people to her. Classified as an ISFJ with an Enneagram Type 6w7, she balances loyalty, empathy, and caution with supportive and adventurous traits.

Physically, she is a young girl with neck-length black hair and brown eyes. Her narrative role emphasizes relationships—particularly with Yakumo, Isshin, and the Gotous—over participation in investigative events.