Although her inmate number is "LB0008", Yoko was not the eighth inmate at the Haguro Prison since the eighth to arrive was Miina Isurugi.
Before she came to Japan she was known as "Hei" one of the many children bought by the Chinese government trained to become assassins. The only person she ever killed was her roommate and friend in the final assassination test. She and nine other children were also experimented on in trying to create Medusas, Hei was the only one whose mirror neurons activiated which later the female doctor discovered girls were more susceptible to implementing murder models.
The female doctor later bought Hei, giving her the name Yoko Kuroki and renrolling her in high school. Later her documents were altered to put her in Haguro Prison.
Unlike all the other girls Yoko is already a skilled fighter and killer, maintaining her skills without the need of her murder model. She can hold her own in close quarters combat, has full training with firearms and even a highly skilled knife thrower. She even has been shown to be highly agile such as when she performed a sneak attack on Chika, another time she performed a complete flip from seating on chair landing on her feet perfectly.
It was implied that she was a trained fighter when she broke the nose of her would be rapist, while the other girls struggled to get away. Her fighting prowess was shown against Yuuji Tani who was lethal Kenpo fighter that could defeat Karen and Katya easily despite their combined efforts. Yoko is a quick thinker using small items such as forks and broken bottles as weapons, even using a small knife ring laced with poison for stealth kills.
In her fight against Tani she was capable of landing quick hard hitting strikes as well as maintain her composure while blocking Tani's, she was even shown skilled in grappling catching Tani off guard for a second. Another time she performed a wrist and elbow lock making a Yakuza shoot himself.
- Among those in the Haguro Prison, Yoko is the last to have her murder model revealed. She was also the first one to experience the Medusa Syndrome.
- The alternative romanization of her name is Youko Kuroki. However, in inside cover of Volume 3 the author has revealed that her name is written as Yoko Kuroki.