S-Ko is the spirit who possesses Zappa throughout Guilty Gear XX. It is possible that S-Ko is the spirit of Megumi, whom Axl left after time-slipping from the 20th century. Also, Burst Encyclopedia clearly says that in her life, she had been betrayed by her lover, and therefore she committed suicide by throwing herself into a well.
While it is not clearly known if she is Axl Low's lover, her hatred of men is clear to everyone with the "Third Eye", apparently an ability possessed by the people that allows its wielder to see her. She seems to like A.B.A, as she spoke of her being Zappa's girlfriend and didn't try to curse her at the first chance; instead having a "civilized" conversation. She also seems to have warmed up to Slayer, who treated her as a lady and offered her a refuge with others like her. Upon the offer, however, she refused and called men scum, in which Slayer responded to her grudge and said he would help her get over it. After defeating Slayer, who clearly goes easy on her, she agrees to go with him under one condition: that she be allowed to go with Zappa. After Zappa woke up, he found himself surrounded by "monsters", whom Slayer said were nice people, but hadn't seen a human in a while; Slayer then went on to explain that it was S-Ko's idea and said he thought it was romantic. S-Ko thinks of Zappa as her boyfriend and is constantly clinging to him regardless of the fact he can't see her (outside of the alternate dimension inhabited by monsters).
- S-Ko as a whole seems to be based on Sadako Yamamura, the main antagonist in the Ring series. Alternatively, she may be based on a Japanese folktale about a woman who is betrayed by her (sometimes depending on the version of the story) lover and thrown into a well.
- Zappa's Instant Kill move even has S-Ko showing the opponent a video almost identical to the one in the movie series.