As her older sister, Sadashi is Kayako's closest companion. Since the death of their mother, Sadashi is the rebellious Kayako’s only trusted family member. Sometimes, Sadashi acts more like a parent, always worried about her sister's safety. In the days when Kayako was in the gang, she participated in more and more violent fights against other gangs, and often returned home with injuries. Sadashi started stalking her secretly, in case Kayako endangered herself. Two years ago, her fear came true: In a deadly conflict, Sadashi was hit in the head, while protecting her sister. Kayako fared no better, and they were both about to die. Their father, Hans, later transplanted their brains into two relic cybernetic bodies, provided by the HOC company.
Sadashi's transplantation seemed flawless at first; she quickly adapted to her new body, standing up and walking. But what happened next took everyone by surprise. For unknown reasons, Sadashi suddenly fell into a rampage. Hans seemed to be prepared, and immediately dispatched a security force composed of cyborg soldiers, to subdue Sadashi, who was out of control. Although these cyborg soldiers were the most elite troops, they were still no match for Sadashi’s relic cybernetic body. The laboratory building was full of soldiers' limbs, torn apart by her. Sadashi was finally stopped by her sister, who was also equipped with a relic body. Sadashi eventually came to her senses, just before breaking Kayako’s neck.
Sadashi was devastated by what happened, as the last person she wanted to hurt was her sister. She asked Hans to place electromagnetic shackles on her neck, arms and ankles. These shackles can create electrical shocks, which paralyze her limbs if she goes into another rampage. This setup has proved effective in the subsequent incidents.
Hans eventually discovered what triggered the rampage: Sadashi's brain was permanently damaged during that near-death event, and the damage was tied to her memory. The relic body has the function of repairing brain damage, and every time the body tries to repair the damage to her brain, it will trigger Sadashi's near-death memory. This extreme stimulation activates the unknown ability of the relic body: when the user's brain is unable to function properly, it will switch into a “rampage” status, fighting back indiscriminately against friends and foes. As a final means of survival, this state will activate the maximum ability of the body. Hans quickly realized the potential of this special function. If the state of rampage can be triggered in a controlled situation, the combat effectiveness of the cyborg body user will be greatly improved. He urged Sadashi to go berserk as much as possible, hoping to find a way to control this function.
Sadashi doesn't like her father's utilitarian motives, but she also wants to find a way to control her rampage, so she can stop herself from hurting innocents. Constantly repeating traumatic memories, Sadashi has slowly begun to gain some control over the berserk function of her body. But at the same time, her personality has gradually changed; she is becoming more reticent, only showing a sense of calm when she sees Kayako.