Yotsuba

Yotsuba (四葉) is a minor character in Toaru Majutsu no Index mentioned in Volume 1. When Kamijou Touma hurriedly calls Tsukuyomi Komoe to ask her about "Mind Hound-related matters" (memory manipulation), Komoe recommends calling in a guest esper who specializes in that field. Komoe then points out Yotsuba, who Komoe describes as a Level 4 esper of the Telepath category that is also a member of Judgment.

Trivia
Ever since her name was mentioned in Volume 1, there's been a fan fascination regarding her true identity. The most popular fan theory has Yotsuba be the character that Touma, Index, and Kazakiri Hyouka meet on September 1 during Sherry Cromwell's attack, as she was a telepath esper and a member of Judgment as well. However, it is still unconfirmed.

Another theory has Yotsuba be the Judgment member that Misaka Mikoto meets during her masquerading in the Toaru Kagaku no Railgun manga.

As of Shinyaku Toaru Majutsu no Index Light Novel Volume 01, Yotsuba has yet to make her actual debut.