Hel

Hel is a member of GREMLIN who first appears in Shinyaku Toaru Majutsu no Index Light Novel Volume 08.

Etymology
In Norse mythology,, is a being who rules over the land 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.

Shinyaku Toaru Majutsu no Index
She appears while making her attack on supply ships at the Pacific Ocean by freezing a vast area of the ocean. She then attempts to eliminate the ships and all those abroad them but is intercepted by Silvia and Brunhild Eiktobel. They successfully protect the ships, and their fight begins.

Queen of Niflheimr
Using the 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.

This apparently lays the base for the next step - she then extracts the 'causes of death' from the area she had frozen, in the form of levitating candle-like flames. She can obtain from these flames information about the circumstances and general background of those who perished at the affected location.

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.