Sōichirō Kiryūin

Sōichirō Kiryūin/Isshin Matoi (鬼龍院 総一郎) is the father of Satsuki Kiryūin and Ryūko Matoi. He worked alongside Ragyō Kiryūin in the research of Life Fibers, but turned against her after she disposed of Ryūko for her apparent incompatibility with having Fibers implanted into her body. Since he knew he would be punished for his betrayal, he taught Satsuki, at the time five years old, about the secrets behind Life Fibers, its research and her presumed dead sister, leaving her with the legacy of the rebellion against the Life Fibers.

According to Satsuki, Sōichirō was murdered by Ragyō (or her operatives) some time after his betrayal, however, he merely faked his death and changed his identity to Isshin Matoi, and raised Ryuuko.