Nao is the younger cousin of Yakumo. She has been deaf since birth and lives with Isshin Saito in a Buddhist temple. She is unable to hear but is able to sense the feelings of those around her very clearly.
Secret Files - Kizuna
It is revealed in this volume that she is not Yakumo's cousin - she is his younger sister from a different mother; Nao's mum was also raped by the man with red eyes. She had a twin brother who was born with two red eyes, however he died not long after being born because Akemi (Nao's real mother) lost consciousness and accidentally dropped him on the floor. Isshin handed in his marriage certificate to Akemi so that he could take Nao in as his daughter. Nao was known to be shy around strangers when she was a baby, however she was easily friendly and open around Isshin and Yakumo.
She becomes Gotou's and Atsuko's daughter after Isshin passes away in volume 6 of the series. They temporarily move into Isshin's old home (the temple he presided over).
