Lucifer

Lucifer (光を掲げる者(ルシフェル)) in the world of Toaru Majutsu no Index is an important figure in Christianity. He is a Fallen angel who rebelled against God until he was defeated by Archangel Michael.

Background
Lucifer the Light-Bringer was said to be an angel that was the closest being to be allowed to sit to the right of God. Since angels are simply perfect tools created by God, Lucifer somehow malfunctioned as a tool, causing in one-third of all the angels, making them rebel against God's will and causing war, becoming a fallen angel. He is later defeated by Archangel Michael created by God to defeat Lucifer, tradition stating that he defeated him with right hand, who then became the ruler of all the angels, and had become a higher being compared to Lucifer even though they were once equals.

Academy City Invasion Arc
Vento of the Front tells the story of Lucifer's fall to Kamijou Touma after discovering Fuse=KAZAKIRI comparing her to a fallen angel that must be destroyed.

Document of Constantine Arc
Lidvia Lorenzetti relates the story of Lucifer to Stiyl Magnus in his interrogation of her, to explain the goals of the God's Right Seat. She references Lucifer's queer position of being at the Right Side of God, which makes him equal to God, despite being an angel, and that God's Right Seat sees a special meaning to an angel being equal to God.

Acqua of the Back Arc
Fiamma of the Right references Lucifer's defeat at the hands of Archangel Michael using his right hand as a significant symbolism that involves his Holy Right.

Toaru Majutsu no Index II Archives 1
Lucifer is the main focus of the audio drama. Here, an institution in the Anglican Church wanted to summon Lucifer, using twin sisters, elder sister Melvina Bailey and younger sister Glencia Bailey, who has a magic circle carved on her back for the summoning. The plot revolves around them being chased by Necessarius because one of them could summon Lucifer. Stiyl was tasked to retrieve the twins and got into a fight with Touma who vowed to protect them. It turns out the older sister wants to summon Lucifer and destroy the world. Stiyl tells Touma that he was told a lie and that they have to stop them from summoning. Touma prevents the summoning ritual with the Imagine Breaker. The failed summoning destroyed the magic circle for the summoning on the sisters' bodies as well as their memories, with the exception of the memory of being saved by Touma. As a result, the Anglican Church deemed them harmless and they were taken in by the Necessarius Women's Dormitory.