Kihara Nayuta (木原 那由他 Kihara Nayuta?) is a character introduced in a Toaru Kagaku no Railgun mini-novel by Baccano! and Durarara!! author, Narita Ryogho. Like her relative Therestina Kihara Lifeline, Nayuta took part in experiments at a young age, but because those experiments ensured the participants' safety, her relatives looked down on her in disdain, which sparked her anger over being called a "flawed item". She had no kids the same age as her to talk to back then, and she talked with Nunotaba Shinobu only a few times. One day, she met in the Advanced Education Department Edasaki Banri and the other Child Errors that Kiyama Harumi was teaching, and became friends with them after a rocky start. Wanting to prove that they weren't "flawed items", Nayuta and her friends aimed to join Judgment, protect Academy City, and become Level 5s, but the experiment that left her friends comatose shook Nayuta, leading her to believe that her friends' teacher, Kiyama, was to blame. Driven by revenge on Kiyama, her own desire to fulfill the dream she and her friends shared, and the naive idea that she could save children from dangerous experiments by taking their place, Nayuta volunteered herself to her grandfather Kihara Gensei. She later learned of Kiyama's actions to save Banri and the others and of Misaka Mikoto stopping Kiyama, which made her want to challenge Mikoto to see if Mikoto was someone to be admired.
Even after her grandfather vanished mysteriously, Nayuta continued experimenting on herself, taking drugs which changed the colors of her hair and eyes as a side effect. One fateful experiment rendered her body 70% mechanical, and she is only alive now thanks to Heaven Canceler's intervention. She donated to Child Error facilities, one of them being Cypress Park, the funds she earned from the experiments she volunteered for, thinking that it would dissuade them from subjecting Child Errors to experiments for money.She is part of a special branch of the Advanced Education Department that is affiliated with Judgment. (Source: Toaru Majutsu no Index Wikia)