Valkyrie

A Valkyrie (ヴァルキリー) is a type of person considered to be similar to a Saint or an Angel in that they act as a bridge between man and God, thus there are many kinds of magic related to Valkyries. This includes ways of sending warriors' souls to Valhalla, ways of recreating weapons that Valkyries used and even ways of "creating" a Valkyrie without the help of Gods. There are limitations with "artificial" Valkyries but occasionally, there are people born with the qualities of a Valkyrie and are much stronger than the artificial ones, like Brunhild Eikdopr. Valkyries are introduced in the Toaru Majutsu no Index: Kanzaki SS and Toaru Majutsu no Index Light Novel Volume SS2 stories.

Background
In Norse mythology, a Valkyrie was a celestial maiden or a term for a human warrior girl. They were said to have the duty of sending the souls of warriors to Odin’s Valhalla in preparation for the coming final battle of Ragnarok, but there were sects that believed that they were not originally angels or spirits or anything else “not originally human”. Apparently, human girls could become Valkyries by wishing for the battle more than anything and being given power by Odin. Basically, courageous girls could become deified by heading down a special path.

There are many theories about what a Valkyrie is and how to become one. One of the theories say that a Valkyrie is not a God, angel, fairy or some other divine existence. Instead they may be just normal human girls who wish to fight more than anything. Once Odin grants her this wish and gives her power, she becomes a Valkyrie.