The Rebel King of Hell
Lucifer was an angel in heaven before he and other angels rebelled against God. They were expelled from heaven, and Lucifer and a third of the heavenly host were thrown down to the pit of Hell. The angels then became demons.
Lucifer became the ruler of Hell and all its denizens. His modern personality owes much to his portrayal in Dante's Divine Comedy, where he is encased in the lake of ice known as Cocytus in the ninth circle of hell along with all those who committed treason against God.
In Shin Megami Tensei
Lucifer's role in the Shin Megami Tensei series is often to be the foil of YHVH, or at least offer a differing alignment perspective to the player. He sometimes takes the guise of a human-looking man named Louis Cypher to act as a sort of guide or watcher, looking in from the outside but nonetheless acting as a participating player in the story.
Shin Megami Tensei: Nocturne
In Shin Megami Tensei: Nocturne, Lucifer appears to the Demi-Fiend as both a young boy and an old man in a wheelchair. As the young boy, he injects him with the Magatama that turns him into a half-demon and takes pleasure in seeing how his actions will eventually unfold. As the old man, he more actively beckons the Demi-Fiend to join his side and consider the power of becoming a full-fledged demon. Should the Demi-Fiend choose that path, Lucifer will directly challenge his power to see if he is worthy.
Shin Megami Tensei: Strange Journey
In Strange Journey, Lucifer appears as a woman named Louisa Ferre, again acting as the Chaos path catalyst for the player character. He appears as a woman due to the game's theme of maternity and subsequent focus on mother goddesses.
Lucifer can only be fused with the Judgment Social Link maxed and with the Main Character at Level 93 or higher. Lucifer blocks Darkness and is weak to Light.