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 their identity.

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 them.

Agitate Halation Arc
During the riots in Academy City's Learning Core involving 7500 "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, reveals that he is pretending to be Number Six, as she and the real Number Six have met before.

In the conclucion 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 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 is introduced as "Eh, who's that Number Six...? Well, forget about it".

Trivia

 * Some fans speculate that Number Six is Aogami Pierce, since pre-memory Touma considered him and Misaka Mikoto as "tough people whom he would have no worries about getting involved" in the trouble revolving around Index.