Sif

Sif (シヴ) is a background character mentioned in Shinyaku Toaru Majutsu no Index. They are a member of GREMLIN, the powerful magic faction that shook the world in its foundations.

Etymology
Sif's name is based on a goddess from Norse mythology, who is a consort of, and a mother to  (Ollerus) whose father is not recorded.

Lance of Gungir Arc
Sif is mentioned in passing, alongside Iðunn, by Marian Slingeneyer after Othinus becomes weakened after forcing the Holistic esper created from Kakine Teitoku's Dark Matter to continue its singing for the production of Gungnir. Here, Marian suggests to Othinus the two GREMLIN members, who currently have no duties, to handle her affairs for her for the production of the lance. Othinus however turns Marian's proposition down and goes off to mend herself.

It is unknown where Sif is during the course of the massive worldwide operation by GREMLIN and their invasion of Tokyo.

VS the World
Sif is mentioned in the conversation between Hel, Jörmungandr and Fenrir at Egeskov Castle with other members of GREMLIN, waiting for Othinus to arrive and take revenge for her abandonment of them, surprising the group, having been part of the rear guard alongside Iðunn. Sif is likely among the pile of scattered bodies when Touma and Othinus arrive at the castle.