Nuada

Nuada (ヌアダ) is a Magic God of Celtic Mythology and a member of the true Gremlin, first mentioned in Shinyaku Toaru Majutsu no Index Volume 13.

Etymology
was the first king of the Tuatha Dé Danann in Celtic Mythology. He lost an arm in battle and was removed from kingship as a result, but was returned to the throne after having his lost arm replaced by one made of silver. His silver arm was the source of his epithet Airgetlám (meaning silver hand/arm).

Appearance
While Nuada's appearance hasn't been described during an actual appearance by him, a fake version of him formed by Sample Shoggoth appears as a light-haired, half-naked young man with war point tattoos covering his entire body and a silver prosthetic left arm.

Magic God Invasion Arc
At some point around the time of the incident instigated by the High Priest, Nuada encountered Kamisato Kakeru and was 'exiled' by his World Rejecter. His silver arm is later returned before Nephthys

Salome Arc
Nuada remains exiled, but his 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 Nuada and the Magic Gods whose appearances were shown for the first time through Yuiitsu's fabrications, given their lack of mention in the story.