Mikoto Misaka is an electromaster esper of Academy City. Her powers are ranked Level 5, and she is considered the third strongest in the city.
Mikoto, a Level 5 electromaster attending a prestigious all-girls Tokiwadai Academy with the reputation of being the third strongest ability user in Academy City, meets Touma Kamijo as he tries to "save" delinquents hitting on her who were unaware of her deadly abilities. He unintentionally insults her in his attempt to persuade the delinquents to stay away from her and she, in her annoyance, discharges a powerful electric shock. Due to Touma's ability to negate the powers of others, however, he is able to walk away unscathed. Upset by this experience, her future encounters with Touma are typically belligerent. Nevertheless, Touma voluntarily assists Mikoto in times of crisis.
Mikoto was conceived by Kiyotaka Haimura, a Japanese manga artist who specializes in character design. Her personality and interactions in A Certain Magical Index and A Certain Scientific Railgun light novels and anime series were done by Kazuma Kamachi, author of both light novel series. In an interview, Kazuma Kamichi revealed that he would most prefer befriending Mikoto out of all the characters of his from the franchise.
While originally created to be a supporting character of A Certain Magical Index, her popularity inspired the conception of a spin-off light novel and anime series, A Certain Scientific Railgun, in which she is the protagonist.
Despite now being the heroine of her own light novel series, she continues to be a recurring character in A Certain Magical Index, oftentimes directly involved with certain story arcs.
The daughter of Tabikake and Misuzu Misaka, Mikoto realized her potential as an esper at an early age. As such, she attended school in Academy City - a place focused primarily on educating and training espers - and rapidly rose from Level 1 to the highest possible Level 5 ranked esper while still a child. She was even requested to lend her DNA for experiments that she was told would help less fortunate people in Academy City.
She earns her nickname, "The Railgun," due to her proven exceptional technical proficiency at utilizing her powers of electric current manipulation in launching projectiles at dangerously high speeds.
Now a teenager, Mikoto attends the prestigious all-girls Tokiwadai Middle School. Recognized to be the third strongest ability user in Academy City, she is also known as "The Ace of Tokiwadai." She dorms with a Judgment (disciplinary committee) member and Level 4 ability user, Kuroko Shirai, who is obsessively attracted to Mikoto. Despite Mikoto's belligerent irritation towards Kuroko's persistence in attempting to force romantic encounters with her, Mikoto oftentimes assists Kuroko with her work at Judgment. (This forms a basis for arcs in the spin-off A Certain Scientific Railgun franchise.)
Mikoto's franchise "origin" and first encounter with Touma Kamijo. She becomes so irritated by how he is unaffected by her electrical discharges that she challenges him to a battle. While Mikoto aggressively attacks Kamijo, he is able to negate all of her attacks. At some point, she unleashes an electrical charge so powerful that it starts a blackout across Academy City. Touma remains unaffected and is able to escape in the chaos.
Mikoto, as a child, thought she was donating her DNA to a philanthropic cause. In reality, she unknowingly donated it to Academy City scientists wishing to harness her ability user prowess to force even greater advances in the powers of others, potentially pushing the strongest ability user in Academy City, Accelerator, to reach an unprecedented Level 6.
They utilize Mikoto's DNA to create large amounts of expendable Misaka clones in a project designated as "Sisters." Enough clones are created such that each one is assigned a five-digit serial number.
To push Accelerator's abilities, the scientists force combat encounters with the Misaka clones, usually resulting in their deaths. Learning of this, Mikoto decides to end the experiment herself by clandestinely destroying all facilities and data related to it. Each time she strikes, however, a newer research facility is erected, gathering fresh data of Accelerator's progress. Aware of the inevitability of encountering the Accelerator and certain that he'll be capable of killing her, Mikoto continues to press on with her attacks, bearing responsibility over the unethical experiment.
Encountering Touma the same time he meets one of the Misaka sisters, however, she unwittingly gets him involved in the ordeal. Eventually, he even discovers a disfigured Misaka sister, forcing him to take the initiative in gathering information on what is happening to the Misaka clones. He eventually confronts Mikoto in her dorm and discovers documents detailing the entire experiment. While convincing Mikoto to not confront the Accelerator alone, he is able to find the next location at which the Accelerator was to battle another Misaka sister. Touma confronts him. Not accompanying Touma, Mikoto knew he would be unable to defeat the Accelerator alone, either. She decides to help him out by enlisting the help of surviving Misaka sisters. Together, they are able to prevent the Accelerator from capitalizing on his ability to control vectors by using Academy City's wind turbines against him and ultimately defeat him, putting an end to the experiment.
Mikoto encounters a young man named Unebara Mitsuki. He confesses his infatuation with her and asks to spend the day. Wishing to avoid Unebara and spotting Touma coincidentally passing nearby, Mikoto quickly snatches him, treating him as her boyfriend in order to stave off Unebara. Touma meets Unebara again while buying drinks for Mikoto and realizes that Unebara is genuinely in love with her. Touma attempts to convince Mikoto to give Unebara a chance, but quickly notices a second Unebara outside. The one he spoke to prior was actually a magician posing as Unebara. The magician, revealing himself to be Etzali, quickly attacks Touma before he is able to put his guard up. With the help of Mikoto, however, Touma is able to withstand the attack and defeat Etzali, who warns Touma that he is getting too involved in the world of magicians and is a threat to Etzali and the magician side of the world, motivating his attempted assault.
Mikoto realizes her feelings toward Touma are becoming genuinely warmer, but she refuses to be open about it.
Rumors of a sound file that do not only improve abilities, but create them for Level 0 nonusers, circulate rapidly around Academy City. Because more and more individuals are actually acquiring abilities due to the sound file, Level Upper, many are abusing them in criminal activities. A Level 2 esper, with enhanced powers due to Level Upper, even attempts to bomb a mall that Mikoto and her friends were shopping in. Due to the efforts of Touma, who was also coincidentally present, no one was actually fatally injured and Mikoto was able to apprehend the esper for her roommate and Judgment member, Kuroko. Later, Kuroko and her friend and coworker, Kazari Uiharu, learn that the Level 2 esper and other suspect Level Upper users have gone comatose.
Eventually, Kazari Uiharu's best friend and Level 0, Ruiko Saten, acquires the Level Upper file and uses it, going comatose herself. With the help of a gifted medical ability user frog-faced doctor known as the Heaven Canceler, Mikoto learns of the source of Level Upper, a researcher named Harumi Kiyama. While Kazari encounters Harumi, she is kidnapped after Harumi realizes her plans were compromised.
Mikoto catches up with the two at a roadblock set up by Anti-Skill, an Academy City policing organization, and battles Harumi. Despite Harumi being able to harness the abilities of the Level Upper users exhibiting the same brain waves due to its networking-like effects, her powers are not enough to defeat Mikoto's natural talent. Even after defeating Harumi, however, the concentrated psychic energy (known as AIM) of all the espers abusing Level Upper eventually amalgamates into the AIM Burst, a powerful entity utilizing the concentrated psychic energy to unleash dangerous attacks that Mikoto cannot even repel. The apprehended Harumi reveals in the midst of the crisis that she wrote a program to negate the effects of Level Upper and Kazari is able to find and play the file. Not only is the AIM Burst defeated, but all the comatose Level Upper abusers are able to regain consciousness, including Ruiko Saten.
Harumi Kiyama pursued her research on AIM fields and conceived Level Upper in hopes of curing students she once taught who were all discarded from society and abused in scientific experiments by her supervisors. Because of a failure during an experiment, they were all rendered comatose. Coming closer to an applicable cure, Harumi, with the help of the Heaven Canceler, attempts to awaken the children. However, her attempts are only met with an adverse reaction in AIM fields by the still comatose children, causing catastrophic events to occur randomly in Academy City and starting the rumor of a "Poltergeist."
While Mikoto, Shirai, Uiharu and Ruiko were accompanying a girl related to the Poltergeist incidents, Erii Haruue, an earthquake strikes the Nature Park they were at. Academy City's Disaster Response organization, Multi Active Rescue (MAR), led by a competent young woman named Telestina, quickly enters the scene and takes the incapacitated Erii Haruue to one of their research facilities.
Due to Erii's relation to one of Harumi's students, Mikoto is able to track the incident back to Harumi once again. While Harumi reveals that she is very close in finding a cure for her children through the product of an older experiment, Crystallized Esper Essence, Telestina enters the scene with MAR and confiscates the children. Mikoto and Harumi, trying to find a way to get the children back, learn that Telestina is actually the granddaughter of the researcher responsible for discovering Crystallized Esper Essence and is related to the abuse of the children through experiments. Telestina herself intends on abusing the comatose children even further in hopes of attaining a Level 6 esper.
Through the combined efforts of Mikoto, Kuroko, Kazari and Ruiko, Telestina is defeated and Harumi is able to finally revive her comatose students.
Mikoto Misaka's base ability is the creation and manipulation of electric charges and currents. She is capable of generating an electric potential difference of up to five billion volts.
Electromagnetism - Mikoto can induce magnetic fields through her ability to manipulate electric charges, and is thus able to maneuver ferrous materials from a distance. She can even amalgamate metals into more lethal forms such as a sword made from iron sand with high speed vibration that cuts like chainsaw. On an even more technical vein, she is able to move herself in otherwise humanly impossible ways - such as along the walls of buildings - due to her mastery of electromagnetism.
Railgun - Mikoto's signature ability from which her primary nickname originates; her ability to manipulate electric currents allows her to emulate a railgun by firing projectiles, typically arcade coins, at velocities of 1.03 kilometers per second.
Hacking - She can manipulate electric signals on any device to the extent of being able to bypass most security measures.
EM Analysis - Due to her ability to manipulate electric charges and current, Mikoto is even able to perceive her own electromagnetic waves and can sense objects simply judging by the movement of her electromagnetic waves in the area surrounding her.
Telepathic Manipulation Immunity - Mikoto's AIM field includes an electromagnetic barrier that could block off direct telepathic manipulation to her brain.
Manual Nervous System Control - She could manually control her nervous system via using her electrical manipulation to directly control the bioelectricity in her body after she was paralyzed with the use of drugs.
Mikoto Misaka had a cameo appearance in the anime series Okamisan, another J.C. Staff production. She appeared in a montage of bride candidate for Chutaro Nezumi in Okamisan Episode 8. She had no spoken dialog.