Jeffrey was a human in Supernatural who loved demons and he always wanted to be one. He was the only human who loved possession. Jeffrey was possessed by a demon who knew of Lilith's location and Sam and Dean tortured him for knowledge on Lilith but the demon escaped before they could get information.
Years later, after Lucifer had been escaped and after the Apocalypse happened, Jeffrey got bored and decided to summon the demon again to become invincible. He wanted possession again and decided to kill women to appease and summon the demon.
Unfortunately, Sam Winchester was being tortured by hallucinations of Lucifer at this point, having been recently freed from Hell, and Lucifer convinced Sam that this world was a mass hallucination. Sam got more evil under Lucifer's influence, and he tracked down Nora, a woman who Jeffrey was going to sacrifice to release his demon friend.
Eventually, although Lucifer got bored of Sam "ignoring" him, Sam and Lucifer agreed to put their differences behind them, and they both teamed up to defeat Jeffrey and his demon. They had a funny discussion about the summoning, with Sam saying "Summoning, but why?" and Lucifer said "Why, to summon demons, dumbass." Eventually Sam found from Nora where Jeffrey was going to free the demon and told Dean, but Dean was knocked unconscious by Jeffrey, and dragged to his warehouse, where Jeffrey boasted about how he'd become even more powerful, and he perfomed the demon summoning, which got done successfully. However, the demon possessed someone else and infuriated Jeffrey, who begged to be possessed again. The demon denied him another chance, laughing at Jeffrey's helplessness. Dean eventually killed Jeffrey when Jeffrey begged to be possessed, but before he banished the demon, the demon laughed at Dean and said Jeffrey would one day return as a demon, stronger than before.