A Gremlin is a creature that originates from folklore which emerged during the early 20th century, primarily from stories told among airmen. They are little creatures, mischievous like imps of older folklore, who sabotage modern machinery, aircraft in particular.
Though similar to the group in the regular world that also bears the name, the true Gremlin differs in that rather than being a fusion of science and magic or an anti-science group, it is an organization that any Magic God from any religion can take part in equally, as a Gremlin is a plain fairy not dyed in the colors of any existing religion. As such, the group is comprised of Magic Gods from various religions, many of whom are remarkable but lacking in some way.
The group is a reconciliation council that was created in order to avoid a struggle over resources between the Magic Gods, all of whom have the power to change the world as they desire but have only one world between them and are all capable of destroying the world, intentionally or not. Though a minority in numbers in the Magic Side, they hold almost all of the Magic Side's total power and thus have a considerable effect on powers formed from the dreams of all magicians.
Though the Magic Gods of Gremlin are not interested in humanity and the outside world, being interested in living happily hidden by themselves, they have a slight concern about the distortions caused by their existence. Regardless of whether they peacefully talk or exchange blows over issues and clashes of opinion, or do nothing at all, their actions can unintentionally influence the outside world and bend the rails of destiny. They are also areas where even they are unsure what to do.
In order to address this slight concern and give themselves a piece of mind, Gremlin wished to obtain a "scorer" (採点者 Saiten-sha?) - someone who would give them a singular set of values and definite directional focus. The scorer would negotiate with them and give them answers from the perspective of one in the outside world, allowing them to score and regulate their power and the distortions it creates. In exchange for giving them piece of mind, they would grant their scorer's wishes and prayers, allowing the scorer to indirectly mimic being a Magic God and influence the world.
The High Priest likened the idea of a scorer to that of security software giving them a 'no threats found' message, a tool that a god uses to judge if their actions are right, similar to Mimir's Head or the Scales of Anubis, a sheath to hold the sword of the Magic God's power, or a seventh path and category separate from the Six Paths.
At some point in the past, Aleister Crowley had the opportunity to become a Magic God and a member of Gremlin but choose to remain human instead. The Magic Gods were connected to the phenomena involved in the early death of his daughter.
Shinyaku Toaru Majutsu no Index
Magic God Othinus Arc
- Main article: Magic God Othinus Arc
Near the end of the events in Denmark, Gremlin interfere with the existing world to prevent Othinus' complete destruction and a potential change in Kamijou Touma which could affect their plans for his future. As they discuss these events, Aleister Crowley penetrates their realm and confronts them. After being provoked by one of the Magic Gods, Aleister summons the Blasting Rod and declares his Magic Name before attacking them.
The confrontation ended with Aleister escaping back to Academy City with a third of his body badly burned, but Gremlin lost their hidden world. Somewhere around this time, the Magic God's subconscious doubts brought about by the situation regarding Touma and Othinus led to the creation of the World Rejecter.
St. Germain Arc
- Main article: St. Germain Arc
Having lost their hidden world, the members of Gremlin are forced to act themselves. Using a method devised by Zombie to split up their existence enough for them to enter the regular world safely, the High Priest, Nephthys and Niang-Niang enter Academy City in order to carry out their work. After being delayed due to the actions of St. Germain, they decide to go after Kamijou Touma but before they could, they are contacted by Aleister who, having obtained their base parameters from their earlier battle and dealt with Zombie while they were preoccupied, substituted Zombie's spell with his own and inserted it into their bodies. After telling them it was high time they remembered what death feels like, Aleister has the crucified Zombie launched into their midst by Kihara Noukan.
Magic God Invasion Arc
- Main article: Magic God Invasion Arc
On December 3rd, the High Priest approaches Kamijou Touma, requesting that he become Gremlin's scorer. On being rejected, he chases Touma through Academy City, causing massive destruction along the way. Nephthys and Niang-Niang also approach Touma and offer to stop the High Priest's rampage but are declined. The High Priest is eventually launched into space using a mass driver but attempts to return using the Arrowhead Comet. However, he is killed by Kihara Noukan before the comet can destroy Academy City. Around the same time, the other members of Gremlin encounter Kamisato Kakeru and are 'exiled' by his World Rejecter, with only Nephthys escaping due to her separated organs.
World Rejecter Arc
- Main article: World Rejecter Arc
With the rest of Gremlin gone, the weakened Nephthys seeks refuge with Touma, knowing that Kakeru would likely go after Othinus as well. While Nephthys was at the Kamijou Residence, the two boys come into conflict not just over the Magic Gods but also the conflict between Patricia Birdway, infested by Sample Shoggoth and Leivinia Birdway, planning to save her through the dangerous Cannibalization technique. When the parasite ended up being eliminated prematurely, Nephthys sacrificed herself to save Patricia from death.
Kamisato Rescue Arc
- Main article: Kamisato Rescue Arc
On December 9th, after Kamisato Kakeru himself fell victim to the World Rejecter's power, the members of the Gremlin beat him up shortly after his arrival in the 'new world', though they didn't kill him. At that point, the Magic Gods were content to stay in the isolated world, where their actions wouldn't damage the regular world, but were keen for the new arrival to leave as soon as possible. Nephthys and Niang-Niang protected him when the others decided to continue the punishment, with the devastating battle being contained within the isolated world.
Processor Suit Arc
- Main article: Processor Suit Arc
On the morning of December 12nd, the Magic Gods received a new toy to play with, in the form of Coronzon who had been shifted to the 'new world' through the machinations of Aiwass and relativistic effects after being launched into space onboard the Windowless Building by Aleister. When the demon managed to escape back to the regular world, Nephthys and Niang-Niang were brought back with it.
- Nephthys (Egyptian)
- High Priest (Buddhism (Japanese variant))
- Niang-Niang (Chinese)
- Zombie (Voodoo)
- Chimera (Greek)
- Tezcatlipoca (Aztec)
- Nuada (Celtic)
- Proserpina (Roman)
- Forgotten God
- Unnamed others 
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Shinyaku Toaru Majutsu no Index Light Novel Volume 10 Epilogue
- ↑ 2.0 2.1 2.2 2.3 2.4 Shinyaku Toaru Majutsu no Index Light Novel Volume 13 Chapter 1 Part 8
- ↑ 3.0 3.1 Shinyaku Toaru Majutsu no Index Light Novel Volume 14 Chapter 1 Part 1
- ↑ 4.0 4.1 Shinyaku Toaru Majutsu no Index Light Novel Volume 17 Between the Lines 2
- ↑ Shinyaku Toaru Majutsu no Index Light Novel Volume 13 Chapter 4 Part 12
- ↑ Shinyaku Toaru Majutsu no Index Light Novel Volume 11 Epilogue
- ↑ 7.0 7.1 Shinyaku Toaru Majutsu no Index Light Novel Volume 12 Prologue
- ↑ Shinyaku Toaru Majutsu no Index Light Novel Volume 12 Epilogue
- ↑ 9.0 9.1 Shinyaku Toaru Majutsu no Index Light Novel Volume 13 Epilogue
- ↑ Shinyaku Toaru Majutsu no Index Light Novel Volume 14 Chapter 1
- ↑ Shinyaku Toaru Majutsu no Index Light Novel Volume 14 Chapter 2
- ↑ Shinyaku Toaru Majutsu no Index Light Novel Volume 14 Chapter 3
- ↑ Shinyaku Toaru Majutsu no Index Light Novel Volume 14 Chapter 4 Part 4
- ↑ Shinyaku Toaru Majutsu no Index Light Novel Volume 17 Prologue
- ↑ Shinyaku Toaru Majutsu no Index Light Novel Volume 17 Between the Lines 1
- ↑ Shinyaku Toaru Majutsu no Index Light Novel Volume 17 Between the Lines 3
- ↑ Shinyaku Toaru Majutsu no Index Light Novel Volume 19 Chapter 4 Part 3
- ↑ 18.0 18.1 Shinyaku Toaru Majutsu no Index Light Novel Volume 19 Between the Lines 4
- ↑ 19.0 19.1 Shinyaku Toaru Majutsu no Index Light Novel Volume 19 Epilogue