Adam Milligan (Jake Abel): John Winchester’s son with Kate Milligan, a woman he met while on a hunt, making him Sam and Dean’s half-brother, one that they knew nothing about until the aptly named episode “Jump the Shark.” He didn’t survive the episode, killed by ghouls along with his mother, and seemed like a random side note for the Winchester family until the angels revealed him as their Plan B after Dean refused to become Michael’s vessel. Adam was brought back to life, but it turned out that he was just bait meant to lure Dean into saying “yes” to Michael. But Dean still refused and Adam ended up becoming the vessel anyway. He met Lucifer, in Sam’s body, on the intended battlefield but got dragged into the cage in Hell when Sam briefly took control. Supposedly he’s still down there, caught between Michael and Lucifer as they brutally fight out their daddy issues.
Chuck Shurley (Rob Benedict): A writer of pulp fantasy novels and a Prophet of the Lord. Chuck started having horrible headaches followed by strange visions. He wrote the visions down, thinking they were story ideas, and published them under the pseudonym Carver Edlund. The visions were of Sam and Dean’s lives on the road, fighting monsters and trying to stop the apocalypse. They were published as the book series “Supernatural,” which had a small but devoted fanbase (much like the show). The publisher went under after the publication of the book where Dean was sent to Hell, or the equivalent of the third season of the show. Sam and Dean discovered the existence of the books and hunted Chuck down, who was very confused by his fictional characters showing up at his house. Castiel stepped in to explain to the brothers and Chuck what was going on: that the visions were prophecies. Chuck occasionally helped the brothers out, including telling Dean the location of the battlefield where Michael and Lucifer were going to fight. At the end of the fifth season, the apocalypse averted, Chuck disappeared into thin air looking satisfied. He hasn’t been seen since. Feel free to speculate why.
Frank Devereaux (Kevin R. McNally): A technical genius who was an expert in avoiding government surveillance and creating fake IDs and documents. He described himself as “bi-polar with delusional ideation.” When he was 26, he came home to find his wife and kids had been murdered. He met Bobby Singer some time after that, who saved his life. Bobby sent Sam and Dean to meet with Frank for his expertise when the Leviathans took on the brothers’ forms and went on a killing spree. He helped them avoid getting caught without having to go into hiding. He also investigated the Leviathans’ activities, keeping the brothers informed, and figured out that the numbers Bobby had given them before his death were one number off from coordinates, which led to a field where Dick Roman was building something. Later, Frank disappeared, presumably dead, and his hard drive ended up in the hands of Dick Roman. When Dick’s employee Charlie (Felicia Day) hacked into the drive, an email Frank had set up in case of emergencies was sent to Sam and Dean, giving them the GPS coordinates of the drive. They were able to convince Charlie to erase the drive so that Dick wasn’t able to get to the information.
Bela Talbot (Lauren Cohan): A wily thief who had a habit of outwitting and annoying the Winchesters, she specialized in trading occult objects for large sums of money. As a child, she had been abused by her father and made a deal with a demon to have her parents killed to end her suffering. Ten years later, her time was up and she tried to get out of her deal by giving the Colt to Crowley, but her contract remained intact. She was then told to kill Sam in exchange for ending her contract, but Sam managed to evade her. In the end, her fate inevitable, she told Dean that Lilith was the demon that held his contract in the hopes that he could take her down.
Kevin Tran (Osric Chau): Another Prophet, he’s in advanced placement and was studying when he was called upon to interpret the Word of God, a tablet written in an ancient language. Being called upon seemed to force him to do things against his will. Blowing off the important test he had been studying for, he drove to find the tablet and stole it from Sam and Dean, but they stopped him before he got very far. They then watched over him while he translated. The tablet revealed how to kill the Leviathans, but then Kevin was kidnapped and forced to translate the tablet for Dick Roman. After Dick was killed, it was Crowley’s turn to kidnap Kevin for his own nefarious plans.