Yotsuba

Yotsuba (四葉) is a minor character in Toaru Majutsu no Index, mentioned in Volume 1.

Index Arc
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 she describes as a Level 4 esper of the Telepath category that is also a member of Judgment.

Trivia

 * 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, but it is unconfirmed.
 * Another theory has Yotsuba be the Judgment member in the Toaru Kagaku no Railgun manga that mistakes Misaka Mikoto for a fellow member.