Soi is a Seiryuu Celestial Warrior. She was rescued by Nakago when she was a child and is his most devoted follower. The true name is Kaen Haku, born under the Star Room. As a Celestial Warrior, Soi possesses Feng Shui related powers allowing her to control lightning, induce electromagnetic fields, but is also skilled in Fhangzang and chi raising techniques. She is 19 years old and was born in Gen, a province near the Kuto border.
Because her family was so poor, Soi was sold when still a child to a brothel and grew up there. When she was twelve, Nakago rescued her and she fell in love with him. While aware that her love was unrequited and he was only using her to recover and heighten his chi, Soi nevertheless continues to serve him faithfully and to the full extent of her abilities.
Soi normally appears as a formidable armoured fighter who speaks like a man, concealing a kind heart and feminine nature. Despite being enemies with Suzaku, she actually shows sympathy towards Miaka Yuuki, particularly out of jealousy towards Yui, and Tamahome, going so far as to state that she envies the strength of their love and revealing to Miaka that she wasn't actually assualted by Nakago (if she had been, she would have killed Miaka). Like Tomo, she has no desire to harm Miaka, but will do anything for the sake of Nakago.
She is killed by Tasuki in a battle between Konan and Kuto in an attempt to save Nakago; she puts herself between Nakago and Tasuki's sword. Nakago then carries her dead body until he passes from the world of the book into the Real World. In the manga, she is seen again by Nakago in "paradise," alongside his mother when he dies.