Xochitl

Xochitl is a female magician from the Aztec Organization. Her first appareance was in Volume 15 of the main series where she is sent to kill Etzali, then considered a traitor to the same organization. But in the timeline, she appears in the Side Story included with the Toaru Majutsu no Index DVD/BDs which is set after Sherry Cromwell's attack on Academy City, where there is a conflict between the Aztec Organization and the U.S. At first, Xochitl threatens to kill Saten Ruiko in order to get her to help with a small task. Later, Saten gets caught up in the conflict and the U.S. side is about to kill her, but Xochitl saves 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, even fighting Misaka in the process. But in the very end, she ignores the organization's orders and teams up with Misaka and the others to solve the problem.

Because of betraying the organization, she is punished and sent to Academy City to kill the other traitor, Etzali, who left them. She meets up with him on one of GROUP's missions and tries to kill him, but fails.

Later in Volume 19, she makes a brief appearance in Chapter 1, talking to Etzali about the current state of the Aztec Organization. It's revealed that she was Etzali's kouhai, and calls Etzali "Onii-chan" again after a long time. After hearing Motoharu comment about Etzali picking up other girls (a reference to Etzali's pursuit of Misaka Mikoto), she gives Etzali death glares and says that he better not have left their organization for something like that.