She is a female magician from the ' Return of the Winged One ' and a member of MEMBER.
She first appeared 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 Arts and Sciences City Side Story (Railgun SS1) 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. Her punishment is being shuffled into the 'dark side' of Magic VS Science conflicts, and had a Aztec grimoire forcibly pushed onto her. Over two-thirds of her body are ground down to dust as the raw materials for the grimore, and it is replaced with artificial body tissues. Later in Volume 19 Etzali is horrified at her punishment, since he himself is a old-hand in terms of the 'dark side' conflicts and forcing a grimoire onto a person is unheard of.
After being sent into Academy City she somehow ended up as part of MEMBER, the team that serves directly under General Superintendent Aleister. The leader of MEMBER, Hakase (meaning professor), knows about her identity as a Magician.
She meets up with Etzali when GROUP decides to help Musujime Awaki when her old allies are targeted by BLOCK, and allows the rest of GROUP to pass when Etzali says he'll stay and deal with this. Before GROUP arrived Xochitl managed to kill off 50 mercenaries hired by BLOCK using her Aztec grimoire. Xochitl and Etzali fought, with Etzali victorious and taking her grimoire when it almost drained away all of Xochitl's life force, binding it to him and swears he'll take on this burden.
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.
Her magic originally is to retrieve a dead person's will, if they have not left one prior to their death. After her failure in USA she had a grimoire that could force her opponent to commit suicide with their own weapons forced onto her. She could also 'take faces' like Etzali by carving the skin off their victims and fashion it into a mask that can change their identity. She might also have magic that can create a 'hammerspace' to store her items, when she took out her 'macuahuitl' from seemingly out of nowhere when she fought Etzali.
In addition to her magic, she is shown to wield a 'macuahuitl', a jade Aztec sword made for crushing and sawing the opponent into two pieces with one blow. Though Xochitl does not know about the saw function and ended up being defeated by Etzali due to that, since the macuahuitl is not a processed metal weapon that can cut easily. In the hands of a trained Aztec warrior Etzali would have been crushed by the physical force, but Xochitl doesn't have that kind of strength. Etzali remarked Xochitl isn't suited for that kind of weapon.
