Real Name: Haku Kaen
Family: Extensive family in Kutou and the youngest who was sold into a brothel as a child.
Place of Birth: Western Kutou
Power: Manipulation through sexual intercourse, control of the elements (lightning and storms in particular)
Seiyuu (Japanese): Tanaka Atsuko(Anime)
Voice Artist (English): Sonia S Fox
Soi is one of several children born to a poor family. Her roots are referred to as "Western" but since she appears so much in Kutou, I think that this must be western territory of Kutou, rather than Sairou - especially considering that Kutou conquered a lot of territories in the past. She was sold to a brothel at age twelve, and fought to escape its clutches, but was unable to do so until she made the acquaintance of Nakago's rival (His name begins with a E - Eichuu?) and discovered her powers of seduction. From then she lived in Kutou's palace as a serving girl, until needed as one of Seiryuu's Celestial Warriors.
I find it hard to hate anything about Soi. She's not a bad person - not even a little bit. She fights for Kutou and Seiryuu for two reasons - one, because she's from Kutou, you imagine - and two, for Nakago. There's nothing she wouldn't do for him, even if he doesn't love her, and this can make her act coldly or harshly towards Nakago's enemies. There is probably some element of jealousy towards Yui for how much closeness she has with the Shogun, but even despite that, her loyalty to him never wavers - in the end, taking a sword for him and sacrificing her life so that he won't be hurt. Her attitude to this is - "at last I was able to be useful to you."
Though she loves Nakago, she willingly impersonates Miaka to seduce Tamahome, simply because it's something that will help Nakago's cause. She sleeps with Nakago often, to improve his life force, though she knows he does not feel for her what she does for him. In essence, her life becomes about Nakago - because of the fact he defended her from abusive men when she was twelve years old. For this reason, she holds the conviction in her heart that despite how cold and cruel he can be, Nakago must have love inside of him somewhere - and she is determined to be there for him, somehow, even if he doesn't want her to be!
That Soi has compassion is clear enough. Although it can't benefit her in any way, her own history as an abused prostitute probably led her to tell Miaka that Nakago had not raped her after all, and that she was therefore still a virgin able to summon Suzaku. She does not really seem to hate the Suzaku warriors - she fights them simply because Nakago is fighting them - and she has sympathy for Ashitare, even though noone else really does. She also steps in to save Suboshi from Tamahome's wrath after the murder of Tamahome's family - whether Nakago ordered her to go is unclear, but it seems more like she wanted to prevent the boy from being harmed.
Soi and Tomo have something of a rivalry because both love Nakago, but if either of them have the upper hand it is probably Soi. Her spirit is so bound to his that she is even waiting for him as he dies and enters the world beyond...and you have to imagine that even cold hearted Nakago is kind of glad to see that....