Ninja

A Ninja (忍者) is a term referring to a type of covert agent or mercenary that existed in feudal Japan. They are in a state of decline in the modern age, despite some ninja clans still existing in modern times with some now operating globally, and are trying to restore their prestige in the world.

Principles
Ninjas were originally bandits that were hired by some powerful military leader. This humble origin allows for anyone to become a ninja. Ever pragmatic, the ninjas uses things from the clothes on their back or even the lack of it to their advantage. Moreover, ninjas always use contemporary weapons for their purposes and disguised them, such as arquebuses camouflaged as staffs and, though is apparently not adverse into using traditional weapons like the uchine. They also apparently prefer weapons that don't stand out too much like an uchine, specifically those that can be concealed or disguised. Finally, ninjas apparently prefer to kill on the first strike and if that fails quietly retreat over a protracted battle. Ninjas don't also lay down their lives for a single goal.

The distinct techniques practiced by ninja are often referred to as ninjutsu (忍術).

Alternative terms often used referring to ninja are Shinobi (忍び) and Kunoichi (くノ一) (the latter referring female ninjas).

Recent History
According to Hattori Hanzou, ninjas are supposed to be in supporting roles as they live in the hidden darkness out of protection and not be in the spotlight. However, as time went on, "pure lines" or families began to spring up like the Hattori and Iga. Hanzou argues that by the time ninjas have "pure lines" they cannot survive in the world. It is likely why the ninja are in a state of decline.

Currently, the Kouga and other ninja clans have been operational in the shadows for a long time and have managed to continue doing so even into the 21st century to varying degrees. Recently, due to changing times, the groups have been looking into Academy City's Power Curriculum Program and Gemstones, and, as such, have been seeking to obtain the means to develop genuine supernatural powers. Though they do not seem to have any awareness of the existence of the Magic side.

Toaru Majutsu no Index Light Novel Volume SS2
On the first Friday of October, Iga ninja Kuruwa caught up with Hattori Hanzou (who had hidden his status in Academy City as a Skill-Out) after pursuing him for half a year and clashed with him over her goal to revive the Iga and Hattori lines, having also been manipulated regarding the Gemstones.

Baggage City Arc
In mid-November, several Kouga ninjas led by Oumi Shuri infiltrated the Natural Selector tournament for the sake of obtaining techniques for their clan, and subsequently got caught up in the battle which engulfed Baggage City.

Dream Ranker Arc
While going through a number of Indian Poker cards, Misaka Mikoto and Kinuhata Saiai experienced ninja-related dreams originating from the Iga ninja Kuruwa, the former experiencing her training under Hattori Hanzou (somewhat different from what she expected of ninjas) and the latter reviewing secret seduction techniques, both to their respective dismays.

Jailbreaker Arc
In mid-October, several Kouga members led by Oumi Shuri, infiltrated Academy City with assistance from Tsurigane Saryou, a Kouga ninja who had previously gone in undercover as a Child Error, and participated in the Second Reformatory's Jailbreak Trial for the purposes of observing Academy City's technology and learning what they could for the Kouga. After the trial, when Tsurigane openly betrayed them and they learned of the apparent circumstances behind her change, Oumi Shuri decided as a senior ninja that Academy City's technology and techniques were something they should never touch again, leaving Tsurigane to be judged as its citizen. Following these events, after escaping from imprisonment, Tsurigane approached Iga ninja Kuruwa for training, hoping to regain Oumi's attention in another way.