Cyborg

A Cyborg (サイボーグ) is a human who has been mechanically modified in order to achieve enhanced performance. They primarily belong to the Science side and presumably only Academy City has enough technology to properly produce them.

Principles
Kamachi Kazuma pointed out that it was Narita Ryohgo (of Bacanno! and Durarara!! fame) who introduced the concept of cyborg into the universe and thus decided to use the concept as a tool in the main series. Kamachi also noted that, in Shinyaku Toaru Majutsu no Index Volume 1, there are two types of cyborgs, one of them being a cyborg that altered her outer silhouette rearranging the inside of her body in order to increase the proficiency of her ability and the other one a "powered suit" that altered the outside of the body and optimized the inner workings of his mind.

The cyborgs from Freshmen were modified for military purposes and are meant to fight, however the first cyborg created by Narita was modified for medical purposes, and her enhancements aren't meant to push forward her combat ability.

While cyborgs are primarily restricted to the Science Side and Academy City, there are a few individuals elsewhere who are referred to as cyborgs but were made through non-scientific means, such as Salome of the Kamisato Faction.

Known Cyborgs

 * Kihara Gensei, the man obssessed with Level 6 and creator of Ability Body Crystal
 * Kihara Nayuta, member of Judgment.
 * Mugino Shizuri, member of ITEM
 * Kuroyoru Umidori, member of Freshmen.
 * Silvercross Alpha, member of Freshmen.
 * Rensa, anti-Level 5s rebellion unit.
 * Kuriba Ryouko (former) (split in two, later recombined)
 * Salome, member of Kamisato Faction