Hel

Hel (ヘル) is a character introduced in Shinyaku Toaru Majutsu no Index. A member of GREMLIN she is deployed in the ocean near Sapain to attack a convoy of ships most likely en route to Tokyo during GREMLIN's invasion there.

Etymology
In Norse mythology,, is the daughter of. She is a being who is appointed by the god to rule over the land of the dead of the same, where she receives a portion of the dead.

Appearance
She is described as a girl wearing a dress that looks as if stained by blood. Her skin has stitches running in every direction. Some of it is youthful, some is wrinkled as an old woman, and some is a discolored dark blue.

Lance of Gungnir Arc
Carried by a small boat in the middle of the Pacific Ocean, she intercepts a convoy carrying supplies, and freezes a vast area of the ocean to stop them. Stepping on the frozen waters, she is accosted by the sailors demanding her to stop advancing. She does not listen, and chants a spell to learn of the location's deaths. She then learns of the gruesome death of Arnold Mackenzie, death by having his head burned in an oven, at which point she takes out a revolver, alerting the sailors there. As the man talking to her to demand her to lay down her weapon, his voice later disappears. A woman then talks to Hel, saying that they have silenced the men who do not know of magic, and mocks her for using such a large spell against them. She then asks if she wants to actually do battle against an expert. Another woman's voice speaks up saying that she is not the type to enjoy fights. Hel asks who are they, to which Silvia introduces herself as well as Brunhild Eiktobel. Hearing this, Hel pulls the trigger of her revolver upon the ship, and though the bullet merely grazes the metal of the ship, it suddenly bursts into flames. She then speaks of how her spell works, but then the fires are put out, and a sound of shattering glass can be heard. Here, Silvia reveals that she has used her barrier magic to stop it, and tells her that if she wants to kill the crew of the ship, she first has to kill her.

Hearing of this, Hel begins to analyze more residual information of deaths in the location, making countless candle-like flames appear before her, signifying each death that has passed there. Here Hel promises to kill Brunhild hearing of her boasting being the queen of the underworld, mocks her for it, as she too have been once called Hel. Thus their battle begins.

she latter appears at Egeskov Castle with Fenrir, Jörmungandr and other members of GREMLIN waiting for Othinus to arrive and take revenge for her abandonment of them. They then notice the crowd part to reveal the arrival of Thor who suggests that they decide on a representative for when Othinus arrives, she asks him on how they will decide on that to which Thor responds that his idea on selection is a battle royal of the members present. She is likely among the pile of scattered bodies when Touma and Othinus arrive at the Castle.

Hel-based magic
As explained by Idol Theory, Hel employs techniques based on her namesake being the ruler of Niflheimr, a realm of primordial ice and cold and the afterlife for those who did not die a notable or heroic death.

First, she is able to freeze large quantities of water in mere seconds. After throwing a simple kitchen knife into the waters of the Pacific Ocean, she had frozen over 100 km of the ocean around her with herself in the center.

Second, she is also able extract the 'causes of death' from the area where she is, though apparently she is required to stand on a solid surface. It is exactly not known if her freezing a large area is required for her spell, though this would make sense based on the legend of Niflheimr. Moreover, it when she finishes her chant, several thousands to even ten of thousands of candle-like levitating flames are said to appear only on the frozen part of the ocean. She can obtain from these flames information about the circumstances and general background of those who perished at the affected location, like their name, age, the events that lead up to their deaths, and the nature of their demise. Hel can then store these 'causes of death' to existing weapons, and by doing so, allow them to kill by that cause of death instead. With this spell, she can strangle with a sword, drown with a train, crush with a blowtorch, and burn with a handgun, the latter of which she applies to completely ignite supply ships she is attacking by using "being burned inside an oven" as a cause of death.

Quotes

 * (To the sailors, from NT 8): "You have plenty of weapons to drive off pirates, blow up cruise missiles, and shoot down planes, but you have nothing ideal for killing a delicate maiden, do you? I suppose I should commend you for hesitating to tear me to pieces."
 * (To Silvia and Brunhild, ibid) “Ten thousand fifty three causes of death have appeared here in all. As expected of a location deeply related to the world’s largest gun nation, many people have died here. …I will use all of these to kill you. And if I kill you, I will use your cause of death somewhere.“