Jakuchō is the second youngest in the Sunobu family, and she was almost murdered by her younger sister Ayano when she was nine years old.

However, even before that her life was insane. When she was three years old, her Father was training her hard behind her Mom's back. She was never taught to speak by him, instead he trained her to use her ability to know what someone is doing in a fight rather than speak. Making her a very powerful weapon. However, at nine she was taken to a facility where people would come and fight her in make believe situations that forced her to use her abilities.

When she was twelve, she was taken somewhere called the Red Room where she trained even further. At twelve years old, she was able to predict someone's movements before they even made them. She never survived as a stand alone assassin, as her Dad trained her to be a weapon and not a soldier.

At seventeen, she was taken by Overhaul when there was an agreement between Hydra and the Yakuza where they would take one of their people to fight for them. Jakuchō was the one that they sent. She had no control or memory over what she was doing, since she was mind controlled and brainwashed to blindly follow orders.

When her sister Ayano came to rescue a girl named Eri from Overhaul, she was tasked with going after and capturing Ayano, since she was Shigaraki's soldier and he wanted her back. The two fought, and Ayano lost against her due to Jakuchō's uncanny ability to predict movements of people, and she brought her to be brainwashed and work for the Yakuza before she would be given back to Tomura.

But, Kirishima had managed to break Ayano's mind control, and Ayano wanted to save her sister along with Eri. Jakuchō was rescued, but she couldn't speak or anything, since she was never taught to. So, Ayano had helped her older sister to speak and to read and write, discovering that she had dyslexia.