L could be for the white eyed Demon, Lilith. L could also be for Dean's ex Lisa, or even Loki, a name taken by Gabriel/The Trickster. But I've chosen the Devil himself, Lucifer.
The main reason for choosing Lucifer is that arguably without him there would be no Supernatural. After all he is Satan, creator of the Demons of Hell and the cause of all evil.
The Archangel Lucifer is the younger brother of Michael and the older brother of Raphael and Gabriel. He was God's favorite but refused to bow down to humanity so was banished from Heaven. As an Archangel Lucifer possesses much more power than normal Angels and Demons, yet he can not just forcefully possess a human, like Demons can, he must have their hosts permission.
When Supernatural began, the yellow eyed Demon Azazel was attempting to free Lucifer, but it wasn't until Sam broke the final seal of Hell in Season 4 that Lucifer walked the Earth again. He set about bringing the Apocalypse to cleanse the Earth of humans.
At the end of Season 5 Lucifer has possessed Sam's body. Sam manages to regain control long enough to throw himself, and his half-brother Adam who was possessed by Lucifer's brother Michael, into a portal in the ground containing Lucifer's Cage.
After his resurrection Lucifer would continue to haunt Sam in the form of hallucinations, and it becomes a major plot point throughout Season 7 - Lucifer taunting Sam that the World he is living in is not real and he is still in the cage imagining this existence. Eventually Sam snaps and has a mental breakdown, Castiel eventually transfers the hallucinations into his own mind and Lucifer fades away.
Although Lucifer took many forms, including the bodies of both Sam and Dean, it was the portrayal by Mark Pellegrino that became a fan favourite, with some people even rooting for the Devil himself...
Next time M is for ?
Previous entries
A is for Azazel - B is for Bobby - C is for Castiel - D is for Dean - E is for Exorcisms - F is for Family - G is for Ghostfacers - H is for Hunters - I is for Impala - J is for John Winchester - K is for King of Hell
