An electricity-wielding middle schooler from To Aru Majutsu no Index, nicknamed "Biri Biri", among other things. Roomate to Shirai Kuroko at all-girls Tokiwadai Private Academy, an upper-class boarding school in Academy City. One of the seven level-5's in Academy city, she's hailed as the "Electric Princess" in Tokiwadai and among the students, and feared as the merciless vigilante "Electromaster" by criminals. Strong and clearheaded, but gets flustered displays immaturity around her acquaintance-cum-rival, Kamijou Touma, the two of whom can never settle their score fairly.
First appearing in Index as a supporting protagonist, arcs focusing on her stick to the Science Side of the city, though she meets Index properly during the course of the story. Events concerning magic and the "Magic Side" often affect her only indirectly.
In the spinoff manga dedicated to her, To Aru Kagaku no Railgun, Mikoto takes the protagonist's role, besting deviant espers and criminals, and unraveling the darker side of Academy City's research and development into the espers. Joining her is, of course, Kuroko, as well as two new(er) characters: Uiharu Kazari, Kuroko's friend and coworker at Judgment, and Saten Ruiko, a level 0 girl. They all develop a considerable amity together and form the main cast of all current installments of Railgun.
- Misaka Imouto: Looks about the same but usually has blank eyes and a head mounted display. Also, while Mikoto has shorts on under her skirt, this one only has striped panties.
- Last Order: Much younger looking than the other two and the only one with an ahoge.
- Misaka Worst: Looks slightly older and wears a gray and white bodysuit or a Vietnamese dress.