Number Six

Number Six (real name unknown) is the sixth ranked Level 5 esper in Academy City. A recurring joke in the series is the mystery surrounding his/her identity. His/her identity alongside the identity of Shokuhou Misaki, the number 5, has been widely anticipated by the fandom.

Daihasei Festival Arc (Railgun)
Number Six is mentioned when Yamane, who is part of a group tasked with convincing the seven Level 5s to hold demonstrations during the Daihasei Festival, tries to track them down, but he is unable to find any trace of him/her.

Agitate Halation Arc
During the riots in Academy City's Learning Core involving 7,500 "heroes", a group of people assume that a boy named Aihana Etsu is Number Six since he defeated them with only his fists and without any apparent ability. Shokuhou Misaki, noticing Etsu, claims that he is pretending to be Number Six, and therefore has no connection to her, thereby indicating that she probably knows the real person.

In the conclusion of the arc, Kihara Noukan states that "the Level 5s gathered in one place", hinting that either Number Six actually was present in Learning Core, that Noukan was simply only referring to the other level 5s, or that Noukan was fooled by Etsu's acting.

Toaru Jihanki no Fanfare
Number Six is mentioned when a pirated broadcast lists off the epithets of the seven Level 5s. Humorously, he/she is introduced as "Eh, who's that Number Six...? Well, forget about it".

Trivia

 * In Volume 3, Kamijou Touma stumbles upon the esper names of the seven Level 5s in documents related to the Level 6 Shift project, but he skips past those names.
 * Some fans speculate that Number Six is Aogami Pierce, since pre-memory Touma considered him over Misaka Mikoto as "tough person whom he would have no worries about getting involved" in the trouble revolving around Index.