Etsu Aihana

He is a boy who, during the riots of "heroes" in Learning Core of Academy City, pretended to be the 6th ranked Level 5. This is backed up by Shokuhou Misaki who claims that he is only acting as the 6th-ranked Level 5.

It is currently unknown whether "Aihana Etsu" is his own real name or the real name of the Number Six.

(Source: Toaru Majutsu no Index Wikia)