She is Sion's best friend. Safu seems to be in love with him, but if he returns her feelings is unclear. The evening before she leaves, she asks Sion a favor, to have sex with her. He promised when she comes back in two years, he will grant her request.
She comes back to No.6 (in the anime, whereas in the original novel she doesn't even leave the city) when her grandmother dies. After talking to Karan she tries to look for Shion on her own, but is captured by the police and taken to Public Order Department and her fate remains unknown for a while. It is later revealed that she has been taken to the Correctional Institute where scientists conduct various experiments on her. Shion and Nezumi try to rescue her, but when they finally reach the Institute, it is already too late as she's been fused with wasps in some sort of system controlling the city. She aids them in their escape and asks them to destroy whole system along with her. She goes out with a smile without regrets.