|Name (Kanji)||木原 那由他|
|Name (Romaji)||Kihara Nayuta|
|Ability Name||Manipulation of other espers' AIM diffusion fields|
|Novel||Toaru Jihanki no Fanfare|
Nayuta's name follows the pattern of Kihara family members having names related to science in mathematics. In Nayuta's case, her name is a Buddhist term that is used to denote an extremely great number or, in another definition, denotes the value of 1060 (or novemdecillion) or 1072 (or trevigintillion).
She has blonde hair which she ties in two pigtails, looks like a elementary schoolgirl, and is often seen wearing her red backpack. Her original hair and eye colors had been changed due to the effects of drugs.
She is a little mischievous, manipulative, and competitive, though not to the extent of her relatives. She is also interested in Level 5s and wants to challenge them and surpass them. She is also quite knowledgeable like her relatives, knowing that both Sogiita Gunha and Misaka Mikoto are Level 5 espers.
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.
Toaru Kagaku no RailgunEdit
She is first seen looking for Level 5s to challenge, but since she is a member of Judgment, she cannot attack anyone without just cause. She later finds Misaka Mikoto kicking Vending Machine 7116 and stealing a drink, and uses this as an excuse to fight her. Using her abilities, she manipulates Mikoto's AIM field so that Mikoto can discharge electricity, which catches the attention of Shirai Kuroko and Sogiita Gunha, both of whom were fighting nearby and decided to go investigate.
When Gunha, Kuroko, and Haratani Yabumi arrive, Nayuta uses her ability on Mikoto to zap Gunha, indirectly sparking a duel between the two Level 5s; both of them, however, show no hint of animosity and are happily testing each other's mettle. The duel's intensity almost puts Nayuta in danger, but she is saved by the sudden appearance of the Tokiwadai Dorm Supervisor. Seeing the Dorm Supervisor easily subdue Mikoto, Nayuta collapses from confusion and fatigue.
When she come to, she finds herself being brought to the hospital on the Dorm Supervisor's back. She wonders if the Dorm Supervisor is a Hound Dog, but the Dorm Supervisor denies having heard of them and reveals that she knows who Nayuta is. She asks how the Dorm Supervisor knows, and learns that, as a Kihara, her donation to the facility the Dorm Supervisor volunteers at prompted the Dorm Supervisor to investigate her. The Dorm Supervisor warns Nayuta that she's only being nice because Nayuta's in grade school; once Nayuta enters middle school, the Dorm Supervisor would handle her like she would any Tokiwadai student. Afraid to make the Dorm Supervisor her enemy, Nayuta silently swears to herself to not pull off another incident like this one for good. Feeling the Dorm Supervisor's kindness, Nayuta falls asleep on the Dorm Supervisor's back, her dream of surpassing the Level 5s renewed.
Her ability allows her to manipulate others' AIM diffusion fields, which is similar to Takitsubo Rikou's power but doesn't require the use of the Body Crystal to activate it. She was able to make Mikoto unconsciously discharge electricity. According to Kihara Amata, if Nayuta trained her ability, she could be deemed capable of defeating Accelerator.
- (Nayuta, explaining to Mikoto): "My family reads the esper's diffusion field flow and uses the gap to attack. Because I haven't learnt that yet, I can only use this mechanical body and my own ability to make up for the lack of senses those amazing people have. They were able to detect the opponent's energy flow just through instincts and experience." -- "The one who taught me all these, Amata oji-san, told me that 'if I train this to the max, there's nothing to be afraid about when fighting that Number One esper in Academy City' and that I can win. But in fact, so what?"
- (Nayuta, talking about Child Errors): "'Child Errors' are just flawed items to Academy City. [...] BUT EVEN IF THEY'RE FLAWED, HUMILIATING THEM LIKE THIS IS UNFORGIVABLE!!"
- In the New Testament Volume 1 Afterword, Kamachi Kazuma acknowledges her existence in the Index series canon, as the reason he had officially introduced cyborgs to the main storyline.