Valkyrie

A Valkyrie (ヴァルキリー) or Walküre (ワルキューレ) is a type of person similar to that of a Saint, a "person with talent", who acts like a bridge between man and God (like angels). Valkyries are derived from Norse mythology, and like their mythological counterparts have incredible powers related to their legend.

Valkyries were first introduced in Toaru Majutsu no Index Light Novel Volume SS2 and are greatly expanded upon in Toaru Majutsu no Index: Kanzaki SS.

Etymology
Valkyrie is a word derived from Old Norse word valkyrja, meaning "chooser of the slain", due to their role in collecting warriors who have died in battle an sending them to Valhalla.

A unique occurrence in the Toaru Majutsu no Index light novels, is the usage of two different katakana spellings for the word valkyrie. To wit, in Toaru Majutsu no Index Light Novel Volume SS2, the light novel uses the English word Valkyrie with the katakana ヴァルキリー, while in Toaru Majutsu no Index: Kanzaki SS, it instead uses the German word Walküre with the ワルキューレ. The reason for this is unknown.

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. Furthermore, upon the hallowed halls of Valhalla, the Valkyrie serves drinks to the warriors and charm them with dancing and have them train.

Principles
There are many theories about what a Valkyrie is and how to become one, as such, there are many sects that believe that they were not originally angels, spirits or anything else "not originally human". Instead they may be just normal human girls who wish to fight more than anything. Once Odin grants their wish and gives them power, they become Valkyries. Basically, courageous girls could become deified by heading down a special path. Based on the legend that "humans can become Valkyries", magicians have attempted in create an easy and simple recipe for humans to become a Valkyrie.

Due to their nature as a bridge between men and the gods, 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, having a proper skeletal characteristics at an organic level, much like how one needs to have a proper skeletal structure to be Chinese Xian, people such as Brunhild Eiktobel.

As both Brunhild and Jeans Slasher can attest, Valkyries are physically strong and are capable of using magic spells that produce special effects that no normal person would be able to produce. Indeed, the narrative states that Brunhild, with a strength of a Valkyrie, can bend a tanker in two with a single punch, in contrast to Saints that have run beyond the speed of sound.

As part of the legend of Valkyries, where they said to occasionally fall in love with people on the Earth, despair at being unable to be with them, and take personal revenge for the sake of the one they love, some Valkyries hold immensely strong emotions. Valkyries such as Brunhild have shown to be willing to fight for the sake of their loved one to the point of destruction, unable to stop themselves.

Artificial Valkyrie
Artificial Valkyries (人工ヴァルキリー) are a magical experiment to turn people into Valkyries after birth by distorting the important parts of the people similar to changing one’s blood type with a bone marrow transplant. The experiment was reportedly stopped, but a new generation was created at some point. One of these people is Othinus, who used several of them to attack Brunhild.

After having her body destroyed by Bruhild, an artificial Valkyrie was shown to be still alive, forming a face using its own blood and vibrated it to create a voice and spoke and would likely reform into her previous shape and begin anew.

Othinus' artificial valkyries wear a skintight riding suit and a jacket tied around their waist by the sleeves. The valkyries were equipped with a western sword on their backs and a hair decoration with bird feathers above their ears.

Saint–Valkyrie
A phenomenon unique to that of Brunhild, is that she is a Saint–Valkyrie. She has both qualities of a Christian Saint and a Norse Valkyrie, but cannot wield both powers at the same time. Both powers were in opposition to each other, which weakens each other's power. Like the waxing and waning of the moon, when one of the other are at their strongest, the other is reduced to zero. Like the moon as well, there is an intermediary period in which both Saint and Valkyrie powers were equal, and as such both powers would be reduced to zero, rendering her an ordinary human for a few days every three months.

Volume SS2: The First Friday of February
The Jeans Slasher is the first valkyrie introduced in the story. Here, she is known to be to be cutting up the jeans of passersby in the streets. Her goal is to use her charm (she wears a bikini armor) due to her goal being to increase her Einherjar and amass as much power as a magic cabal of her own. Kanzaki Kaori meets her near the Jeans Shop Owner's store after she attempts to cut her jeans. After clashing blades with Kaori, she uses her magic to take control of the Jeans Shop Owner with 9 girls, though Kaori is able to knock some sense into him.

When Kaori asks why she only cuts gentlemen's jeans, she says that it is her specialty for the warrior's reception is humiliation play, sadism. Kaori, enraged by the queer and perverted nature of the woman, can be assumed to have beaten her.

Kanzaki SS
In this sidestory, Kanzaki Kaori encounters the Valkyrie-Saint Brunhild Eiktobel who was carrying out attacks against five Norse cabals in revenge for their destruction of the ones she cared about out of hatred for her mixed nature. She also had a hand in the other incidents involving Norse mythology that Kanzaki had dealt with, part of her plan to acquire Gungnir to achieve her goals of revenge and saving Ceillier Flatley. After being tracked down, she fought Kanzaki with the incomplete Gungnir but lost.

Baggage City Arc
Brunhild Eiktobel is confronted by a group artificial Valkyries working for GREMLIN and draw her attention with the fighting to lower her mental defenses to draw out information on Gungnir. She is later helped by the arrival of Silvia and Leivinia Birdway.

Known Valkyrie
Known Valkyrie seen in the series thus far:
 * Brunhild Eiktobel
 * Jeans Slasher
 * A number of unknown artificial Valkyries

Trivia

 * Valkyries are the primary subject of Kamachi Kazuma's The Circumstances of Waltraute's Courtship, where the main heroine, a blond Valkyrie, tries to deal with the young male protagonist, who is in love with her, while various shenanigans from other deities and Norse beings get in the way.