Theodosia Electra

Theodosia Electra (テオドシア＝エレクトラ) is a Necessarius magician who uses Norse mythology as a base for her magic.

Appearance
She has an appearance of around 40 or possibly a little younger. Her hair is a mix of blonde and silver, but it is simply because her hair was damaged. She wears a baggy white top and faded white jeans.

The First Friday of February
Kanzaki Kaori tells the Jeans Shop Owner that she got her information on the Jeans Slasher's actions having the smell of Norse Mythology on them from Theodosia who investigated the incidents.

Toaru Majutsu no Index SS: Stiyl
Theodosia received orders from Necessarius to eliminate Patricia Birdway, but she realized that the girl didn't know anything about magic and was completely innocent. This causes Theodosia to temporarily betray Necessarius to protect Patricia, causing a fight between her and the next Necessarius magician sent after Patricia and her, Stiyl Magnus. After defeating Theodosia, Stiyl realizes that the mission was wrong, leading him to side with Theodosia to protect Patricia.

Theodosia later helps Stiyl undo the mental binding cast on Patricia by Richard Brave to slow them down. When he catches up to them, Theodosia and Stiyl try to fight him, but fail due to Richard's Lævateinn's power. Theodosia and Stiyl split up and go their separate ways, with Theodosia carrying the rune epithaph and Stiyl escorting Patricia.

When he catches up to him and Patricia, Richard Brave shows the rune epitaph to Stiyl, making him believe he had killed Theodosia, but it's later revealed that she had faked her, taking advantage of Lævateinn's overwhelming power to cover up the lack of a body. Theodosia reappears to help Stiyl against Richard Brave's reinforcements, accompanied by Leivinia Birdway and the Dawn-Colored Sunlight forces. Theodosia was later present for the conversation between Stiyl and Leivinia about her cabal and her goals.

British Royal Family Arc
During the Anglican Church's preparations previous to the battle of Buckingham Palace, one of the nuns reports that Theodosia Electra's group has arrived.

Abilities
Stiyl comments that she constantly changes what main spell she uses, because she is either very talented or just an idiot who gets tired of things quickly. Though no matter the changes it always remains a Norse magic based spell. When he meets her she says she's using spells based on the fairy tale of the Little Match Girl. However, shortly after they start fighting Stiyl realizes she's using her matchsticks and matchbox in a spell based on Skírnir’s Staff (スキールニルの杖), a spiritual item with four runes carved into it, based from Norse mythology from Iceland. She was able to combine the flames used to protect treasure and the staff used to steal treasure by optimizing the original pattern for the layout of the runes. The spell has several defensive uses:
 * Space manipulation: The surrounding area is extended, spreading in the same way darkness is pushed away by the light of Theodosia’s match. Theodosia can also manipulate space to put distance between herself and her enemy so that their attacks don't reach her.
 * Hand grenade: Theodosia throws a match at the opponent like a grenade. The match explodes, creating a series of chain explosions to make a fire wall. She can further manipulate this fire wall to attack the enemy by lighting a second match.
 * Headwind: Theodosia lightly waves her matchbox back and forth making a clattering noise. An intense headwind starts blowing, preventing the enemy from moving forward and also blowing away flames.

Other spells

 * Visual data display: Shows information to the person lights it for as long as the match is lit.
 * Drill of Rati (ラティの錐): Based on a story about Odin, it functions as a charm that will allow one to sneak into difficult places and to escape from them. She has smashed it into a powdered form so she could carry it around with her, to make the person cornering her on the edge of flinch by throwing it to their face, practically turning it in a way to revive herself from the dead.
 * Room concealing: Theodosia is capable of preparing a spiritual item that controls the protection and release of treasure, using rune magic to conceal the existence of a room. Unless the person trying to enter uses the rune corresponding to the spell, the door and even the room itself would not exist and thus they won't even be able to try and break into it. However, the seal disappears if Theodosia is defeated.