Jeffrey

Jeffrey was a human that was previously possessed by a demon in the season seven's episode Repo Man.

4 Years Ago
Jeffrey was possessed by a demon that knew Lilith's location, and Sam and Dean tortured him for information. The demon refused to give the brothers information, the demon lets Jeffrey out momentarily to stop Sam and Dean from hurting him. The Winchesters explain to him that they need information from the demon and to get it they will have to inflict great pain and injury. Jeffrey gives them full permission, the demon returns, and Sam and Dean extract the information and exorcize the demon. Sam and Dean drop Jeffrey off at the emergency room and Dean reminds him not to say anything. Over the years, Jeffrey missed the demon and got involved with alcohol, but was saved by a rehab counselor.

Season 7
Jeffrey got the idea to summon the demon back, which prompts him to visit Nora asking for a summoning spell. She turns him down, so Jeffrey kidnaps her son from his dorm and blackmails her into giving him a spell. The spell doesn't work and Jeffrey sends Nora her son's ear, demanding another spell. Nora finds another one that requires the blood of the person who exorcized the demon. So Jeffrey begins killing women in order to draw Sam and Dean out. He also makes sure that Nora send Sam and Dean to him.

Sam and Dean do follow the trail and visit Jeffrey in search of the demon. Sam stays to watch a potential victim, and Dean goes with Jeffrey. Jeffrey takes Dean to a warehouse and knocks Dean out. He reveals to Dean that he loved being possessed and plans to summon the demon again. Jeffrey performs the summoning, but the demon goes into Nora's son instead of Jeffrey. Jeffrey asks the demon to possess him again, but the demon refuses. As Jeffrey is about to attack again, Dean shoots Jeffrey.

Trivia

 * Jeffrey is the first human on the show to actually enjoy being possessed by a demon.
 * Based on comments made by the demon that previously possessed him, Jeffrey's soul likely goes to Hell and will eventually become a demon himself.