Soi is an interesting character. She isn't really a bad person. She just fell in love with the wrong man. She will do whatever Nakago commands. She also had a tragic childhood. She was sold into prostitution by her own father. Nakago saved her (when he found out that she was a Seiryu warrior) From that point on she would follow him to the ends of the earth, even if he never returned her feelings. I think her life is sweetly tragic.