Proserpina

Proserpina (プロセルピナ) is a Magic God of Roman Mythology and a member of the true Gremlin, first mentioned in Shinyaku Toaru Majutsu no Index Volume 13.

Etymology
is the Roman goddess of the Underworld, her Greek equivalent being. A daughter of the agriculture goddess Ceres, she was abducted by Pluto, god of the Underworld, to be his wife. Her distraught mother searched desperately for her daughter, letting the crops wither away in her grief. Jupiter eventually had Pluto release Proserpina but because she ate the food of the dead, she couldn't completely return to the world of the living. As a result, she spends six months with her mother and six months with her husband, resulting in the changing seasons.

Appearance
While Proserpina's appearance hasn't been described during an actual appearance by her, a fake version of her formed by Sample Shoggoth appears as a young woman in a black Western mourning dress with a hood and veil of the same color.

Magic God Invasion Arc
At some point around the time of the incident instigated by the High Priest, Proserpina encountered Kamisato Kakeru and was 'exiled' by his World Rejecter.

Salome Arc
Proserpina remains exiled, but her likeliness was used in a bluff by Kihara Yuiitsu while fighting Kamijou Touma, Salome and Misaka Mikoto, making use of Sample Shoggoth and an attenuated St. Germain virus to make imitations of the Magic Gods and their magic, to make them think she could draw out the Magic Gods and their power after stealing World Rejecter from Kamisato Kakeru.

Character Art Design
According to Haimura, there weren't any particular details focused on in the designs of Proserpina and the Magic Gods whose appearances were shown for the first time through Yuiitsu's fabrications, given their lack of mention in the story.