Freyja

She is a magician, a strange system that is born out of the relationship between a mother and child, the latter being a fetus whom from the womb controls her mother's body and mind.

She is present during the GREMLIN invasion of Tokyo, and delays Kamijou Touma in his quest to stop Othinus. After her defeat at the hands of Touma and Index, the magician known as Freyja ceases to exist, though the mother and child live on.

(Source: Toaru Majutsu no Index Wikia)