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 Aihana 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 Aihana 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 Aihana Etsu.

In the conclusion of the arc, Kihara Noukan states that "the Level 5s gathered in one place", hinting either that the real Aihana 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
Aihana 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", Aihana silently toys with the phone.

St. Germain Arc
Following the events at the Dianoid, during which Kanou Shinka made use of Aihana's name, the real Aihana speaks with Yokosuka, who has been assisting Aihana (hoping that he could learn how to beat Sogiita Gunha) with their plans of helping people solve their problems by allowing them to use Aihana's name as the sixth ranked Level 5 on Aihana's behalf, Yokosuka having passed the name on to another.

Christmas Day Arc
On Christmas Day, Aihana Etsu approached the heavily injured and dying Kamijou Touma, who was attempting to leave hospital to stop Anna Sprengel, despite his condition. Though they had negative view of Touma and had avoided making contact before then, Aihana apparently deemed supporting Touma with their power the best choice for fighting R&C Occultics. They attempted to have Touma take on their name and grant him power like they usually did with others, but Touma rejected their 'offer' and called out their attitude. Both unwilling to budge, Aihana stood in Touma's way and was quickly knocked out.

Daihasei Festival Arc
The Number Six was mentioned when Yamane, who is part of a group tasked with convincing the seven Level 5s to hold demonstrations during the Daihasei Festival, tried to track them down, but was unable to find any trace of them.

Toaru Jihanki no Fanfare
Aihana 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".

Abilities
Aihana's ability allows them to give others power and make them into their 'ideal self'. The exact mechanisms at work are not entirely clear, but by having their target accept their offer and picture their ideal self, the version of themselves that they wish to be, Aihana can apparently make them become that ideal self, granting them the power they wish for. It has been compared to instantly transforming anyone into a protagonist without any annoying training or levelling up.

The process is affected by several factors, one of which is the target's imagination. Aihana generally does not give suggestions as they would narrow the imagination. According to Aihana, although they can give someone power, they can't say they actually 'creates' the power. It may also be limited by certain external conditions. Aihana lends people the name and identity of Aihana Etsu when aiding them, either directly or indirectly, by themselves or through a proxy. Whether this is a necessary part of the empowerment process is not fully confirmed.

When trying to get Kamijou Touma to accept their offer, Aihana gave an example of what their power might be able to accomplish - pointing to the whistle which Touma had received from Shokuhou Misaki, which and whom he could no longer remember. Aihana claimed that if Touma wished to know the meaning behind the whistle or was interested in recovering his lost memories, he could do it as long as he arranged the necessary conditions in the city. Aihana also claimed that it was more convenient than what Rensa had.

The nature of this ability is not one suited for direct combat, meaning Aihana has to use or work with others to achieve their goals. When forced to step outside of their safe zone and face an injured Kamijou Touma in a physical clash, they were easily knocked out.

When Aihana approached the ravaged Kamijou Touma, the latter experienced audio and visual distortions in his perception of his surroundings. As he was descending the stairs (which seemed not to change no matter how many flights he went down), he felt a sensation described like walking on fluffy clouds, with light blurring and the deafening noise around him becoming distorted. When Aihana appeared on the landing before him and spoke, Touma was fairly sure they had not been there before and he could only see a vague outline, like a scene filmed through an out-of-focus video camera. At the same time, the surrounding noise disappeared. When he tried to slip past and descend the next flight of stairs (not seeming to make progress), he found Aihana waiting for him on the next landing (even after placing his right hand to his head). After Aihana's introduction, the visual distortions spread out with the walls and floors (which had calming colors for the benefit of patients) flashing with bright colors and stairs starting to bend and undulate. The illusion disappeared after Aihana was knocked out.

Aihana's voice during this encounter was described as a genderless soprano, likely intentionally modified to make it difficult to discern with a distortion sometimes running through it, which dropped terrifyingly deep when angered.

Trivia

 * In , 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 ', there was confusion over whether the name "Aihana Etsu" was the Number Six's real name or just the impostor's name. In ', Yokosuka confirmed it as the former.
 * Aihana uses the first-person pronoun "boku".

Quotes

 * "Now, picture the you that you wish to be. Picture your ideal self.”[...]“You can be that version of yourself. Now, I will lend you Aihana Etsu."