Yoko Yubari was the sister of a yakuza crimelord of East City, that was killed by his associates when she was a youn girl. Her life was saved by Aleph, the woman that would become the head of the secret organization known as the Company. So Yoko devoted her life to Aleph and started following the way of the samurai.
She became part of the team of the Empty Men, a group of men and women with “nothing to lose”, used as a special squad by Aleph. Among them she took the codename of Idea, the others being Act, Motion, Reality and Shadow.
As she was forced to kill her lover and companion Motion during a mission, she swore revenge against Aleph and the whole Company.