Aihana Etsu

Aihana Etsu (藍花 悦), also referred to as the Number Six, is the sixth ranked Level 5 esper in Academy City. A recurring joke in the series is the mystery surrounding their identity.

Etymology
The three kanji for Etsu's name mean "indigo", "flower", and "pleasure", respectively.

Agitate Halation Arc
During the riots in Academy City's Learning Core involving 7,500 "heroes", a group of people assume that a boy claiming to be Etsu is the Number Six since he defeated them with only his fists and without any apparent ability. Shokuhou Misaki, noticing him, claims that he is pretending to be the Number Six Level 5, and therefore has no connection to her, indicating that she probably knows the real Etsu.

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

Magic God Othinus Arc
Etsu officially debuts in person in this arc, though their features are kept nondescript and they are referred to as the Number Six rather than by name. When Academy City sends an email to the seven Level 5s "requesting the death of Magic God Othinus and Kamijou Touma who is traveling with her", Number Six silently toys with the phone.

St. Germain Arc
Following the events at the Dianoid, during which Kanou Shinka made use of Etsu's name, the real Etsu speaks with Yokosuka, who has been assisting Etsu with their plans of helping people solve their problems by allowing them to use Etsu's name as the sixth ranked Level 5 on Etsu's behalf, afterwards Yokosuka has passed the name on to another.

Daihasei Festival Arc
The 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.

Toaru Jihanki no Fanfare
Etsu is mentioned when a pirated broadcast lists off the epithets of the seven Level 5s. Humorously, they are 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 the Number Six is Aogami Pierce (thus making "Aihana Etsu" his real name), since pre-memory loss Touma considered him and Misaka Mikoto "tough people who he would not have to worry about getting involved" in the trouble revolving around Index.
 * After the release of NT Volume 7, there was confusion over whether the name "Aihana Etsu" was Number Six's real name or just the impostor's. In NT Volume 12, Yokosuka confirms it as the former.

Quotes

 * (While toying with the phone, from NT Volume 10): "..."