She's a magician from the Return of the Winged One organization and a member of MEMBER, the elite team serving directly under General Superintendent Aleister.
Xochitl first shows up in Volume 15 as an assassin sent to kill Etzali, her former comrade who'd defected. But chronologically, she actually appears earlier in the Railgun side story, where she's caught up in a conflict between the Aztec Organization and the U.S. over the Stargate Project. After initially threatening Saten Ruiko to force her cooperation, Xochitl has a change of heart—she saves Saten from U.S. forces and eventually defies orders to help Misaka and the others stop the operation.
That betrayal came with brutal consequences. The organization punished her by forcing an Aztec grimoire into her body, grinding down over two-thirds of her flesh to use as raw material and replacing it with artificial tissue. Etzali, a veteran of the "dark side" conflicts himself, is horrified by this punishment when they meet again—apparently forcing a grimoire onto someone like that is extreme even by their standards.
After being sent to Academy City, she somehow ended up recruited into MEMBER. Hakase, the team's leader, knows her true nature as a magician.
Her original magic let her retrieve the will of the dead. The grimoire she was forced to carry compels opponents to commit suicide using their own weapons. She can also take faces like Etzali does—literally carving skin to fashion identity masks—and possibly create some kind of pocket dimension to store items.
She carries a macuahuitl, a jade Aztec sword designed to crush and saw through an opponent. The problem? She doesn't actually know about the saw function and lacks the raw strength to properly wield it, which is why Etzali beat her. The weapon needs a trained warrior's power to be effective.
By Volume 19, she's reconnected with Etzali, calling him "Onii-chan" again and clearly unhappy about his romantic interests elsewhere. Their relationship hints at a shared past—she was actually his junior in the organization.
Content compiled by AnimeList.moe from publicly available sources.
