Xochitl

Xochitl is a female magician from Aztec Organization, the same as Quetzal. Her first appareance was in the Volume 15 of the main series where she is send to kill Quetzali. But in the time line she appeares in the Side Story included with Toaru Majutsu no Index DVD/BD's which is set after the attack of Sherry Crowmell to Academy City, where there is a conflict between the Aztec organization and the U.S. At first, Xochitl threatened to kill Saten Ruiko in order to get her to help with a small task. Later, Saten gets caught up in the big mess and the U.S. side was about to kill her, but Xochitl saved her. The objective of the organization was to destroy the Stargate Project island and the facilities on it, and Xochitl was assisting with the plan, and even fought Misaka. But in the very end, she ignored the organization's orders and teamed up with Misaka and others to solve the problem.

Because of betraying the organization, she was punished and send to Academy City to kill the other traitor, Etzali. She appears again in vol. 15 and 19 of the main series, but that's not for this thread.