Safu

Safu is an extremely intelligent, stubborn and caring girl who lives in Chronos with her grandmother. She has been Sion's best friend since they were children, but lost contact to him after he moved to Lost Town. Safu seems to be in love with him, but if he returns her feelings is unclear. The evening before she leaves, she visits Shion and asks him for a favor, to have sex with her. He promises when she comes back in two years, he will grant her request.

[Spoiler] 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 Elyurias and no longer possesses a physical body. She aids them in their escape and asks them to destroy the whole system along with her. She goes out with a smile without regrets. [/Spoiler]