When Sam was a baby his mother was murdered by Azazel the yellow-eyed demon. From that point on Sam’s dad became a hunter that went after supernatural beings hurting humans and started to hunt clues to the death of his wife. Sam grew up under the loving care of his big brother, Dean, even if his father was always busy hunting things. As a young child, Sam had no idea the supernatural existed, even though his brother did. Instead, he resented his father's constant absence while he was growing up. He looked up only at his brother, but never quite understood how Dean could feel respect and admiration towards their dad. Another figure that would become like a second dad was Bobby a hunter that was a friend of his dad. Dean was the one who told him the truth, and sometime later when he was old enough, his father started to train him to become a hunter. Much to the chagrin of the young Sam.
Sam started to rebel and question his dad. He felt his dad was too demanding and that he was unfairly placing pressure on him to follow in his footsteps. Finally, when he was old enough he left his dad.. and by extension his older brother. He needed to find himself, to live away from the crazy expectations of his dad, he needed to have a normal life. He managed to enter Stanford, and his new life as a normal person began. He had friends, a girlfriend and did the normal thing for a while. He was living with his girlfriend, Jessica, when his brother knocked on his door telling him their dad was missing. Even though Sam had sworn against hunting ever again, he allowed his older brother to persuade him to go with him. Soon enough he was back on the road, with his brother.
The case their dad had been working as a woman in white, a ghost that would go after unfaithful men tempting them and then killing them. The both of them finally lured the ghost back to her house where she was vanished by the ghosts of her children that she had murdered. Together they managed to guess their father went to Blackwater Ridge. Sam had decided to get back to his life now that he had helped Dean get a useful clue. However, shortly after his return… Jessica was murdered in the exact same way his mom had been and he walked in just to see her body pinned on the ceiling burning. Dean rescued Sam from burning himself to death in his shock. After that Sam wanted revenge, he wanted answers. They started to hunt together, trying to find their dad and know more about the murderous creature that killed both, his mom and Jess.
Every little clue seemed to bring them closer to an answer. However, as they traveled weird things started to happen to Sam. He developed abilities that normal people did not have, like precognition and seeing things in his dreams. He wasn't really sure what was going on and it scared him. After traveling for so long learning demons were real and facing one of them, Meg, persistently, they finally were reunited with their dad. He had some kind of plan to face the demon that killed their mother. Part of the plan involved a gun, the colt. This gun could kill anything but, 5 beings. But, between Meg, the demon interfering and their dad being possessed by the yellow-eyed demon. Everything went wrong, in the end, Dean ended up on the brink of death and John struck a deal with the yellow-eyed demon, Azazel, to save him.
He gave away the colt and his soul. After that point, Dean and Sam were left to grieve and to continue hunting hoping to avenge their parents' deaths. Eventually, it became clear that Sam was a part of some kind of twisted master plan the demon had. Even then, Dean was extremely reluctant to accept Sam could be dangerous. Sam wasn't so sure he wouldn't become evil, in fact, the prospect of lacking control over himself scared him. Sam was kidnapped and very quickly he knew why. He was with some other "special kids". All of them there for the same purpose; to kill each other. Azazel egged them against each other. The winner would lead the armies of hell. Sam did his best to try and stop the craziness.
After much struggle, in the end, it seemed like Dean had found him, for a minute everything seemed alright and the brothers were reunited. But, just as he walked towards Dean Sam received a stab in his back. Sam died in his brother's arms. Sam was dead so he had no saying or inference in what his brother did next. Dean went and made a deal he gave away his soul; in return, Sam was brought back to life. He was a little suspicious, but having a problem with Azazel and the special kid that killed him trying to open the gates of hell he was distracted long enough that he would not ask too many questions. They went and did the heroic hunting thing. Sam ended up killing the last survivor special kid that had killed him. After finding out what Dean did to save him. He decided he wanted to save his brother. They spent the next year trying to get Dean out of the deal.
Along the way, a new demon introduced herself to them. Ruby, had a demon-killing knife. At first, the both of them were kind of wary of her after all, they spent the better part of looking for their dad dealing with Meg, the daughter of Azazel. The experiences with Azazel and Meg showed them how terrible demons could be. Their knowledge of demons had always pointed that there were no good demons out there. However, when Sam heard Ruby could be able to help him break Dean's deal he eagerly wanted to believe she could help. Lilith, a white-eyed demon would be their target as she was both a threat and the current holder of Dean's contract.
They made no progress and Ruby eventually admitted she had no way of breaking the deal. Things got worse when Ruby's vessel was possessed by Lilith and she sent her hellhounds to rip Dean apart. The hell hounds surely found Dean. Dean died and his soul went to hell. Sam thought he was about to join his brother in death when Lilith tried to hit him with her white light, but both of them discovered he was immune to her demon powers. Lilith vanished. Bobby Singer really wanted to give Dean a proper hunter's send-off and burn his body. However, Sam didn't want to lose his brother. He believed he could bring him back making a deal to resurrect him. Sam convinced Bobby and they instead buried his body. Sam tried to make a deal to get Dean back. Nevertheless, no demon wanted to make such a deal. In his frustration, he killed the first crossroads demon that said "No". Sam was heartbroken.
His brother was really gone and he couldn't save him. He started to drink, a very much constantly drunk Sam could not help Dean. But, if he couldn't save Dean at least he could avenge him. He started to track demons, to hunt them down his plan, simple, to kill Lilith or wreck her plans however he could. He almost got himself killed in the process, but Ruby saved him. She convinced him to work with her, to accept her help. Ruby and Sam started to work together and to have sex. Eventually, he too accepted to drink her blood to increase his control over his powers. Somehow, unbeknown to him Dean was brought back to life. Sam was surprised (and for a moment thought that wasn't his brother) and happy that his brother was back. They hugged and because Ruby pretended to be a girl Sam had seduced for one-night stand she left.
Sam told Dean a lie “You said don’t go that road so I didn't go that road.” A lie that would come back to bite him in the ass later. They started to work together again picking up cases. Not long after that, Sam was able to hear from Dean all about Castiel the angel that pulled Dean out of hell on Heaven's orders. San missed the encounter at the barn because he was too busy sneaking out with Ruby. Sam totally drank the kool-aid because he thought the angels gave Dean a real second chance, they would find out that was not the case. Lilith was breaking 66 seals to raise Lucifer; the brothers did their very best to stop her. Sam's facade of not using his powers fell quite fast when Castiel told Dean where to find him.
Luckily their argument was interrupted by a case and on the drive, back after killing the rougarou Sam told Dean he would stop using his powers. They kept trying to protect the seals, but soon enough, Sam was forced to use his powers again. As a part of the 66 seals, witches planned to raise the Halloween demon, Samhain. During their struggle to prevent the breaking of that seal, Samhain was brought back. The two brothers separated briefly. Sam tried to attack Samhain using the demon-killing knife, but the blade fell from his hands. Cornered, he used his powers to exorcise the demon. Sam's resolution not to use his powers was falling apart. Later on, Ruby appeared before them, once again with information. She spoke to them of a woman named Anna Milton whom she has heard was wanted by the demons.
Apparently, Anna could tune in angel radio in her head, for that reason she knew about the Winchester brothers and she too knew angels didn't like Sam. Not only demons wanted her, as it turned out Angels too were after her. They managed to get away, with their help she was able to regain her memories of being a fallen angel. The brothers and Ruby worked on a plan that helped her gain her grace again. After that, Dean finally told Sam that he remembered hell, all of it. Sam also learned that for him it wasn’t 4 months but, 40 years. Alistair would torture him and offer to let him off if he took to torturing other souls in his place. He told Sam his will was broken after 30 years of enduring torture and then he tortured other souls.
Sam wasn't sure what to say other than to be shocked. A part of him started to doubt Dean, but he kept those thoughts for himself. Sam started to experience even more doubts and resumed constant contact with Ruby. Part of him was drawn again by the possibility of ending the threat of Lilith, the breaking of the seals, the escape of Lucifer from his cage, and the apocalypse. He wanted the seemingly endless fight to be over. For that reason, Sam started to practice using his powers again. Sam was certain Dean would never approve of his actions so he again tried to keep it a secret. Sam did his best to keep Dean from knowing about him maintaining contact with Ruby, but even Sam could tell Dean was starting to notice he was hiding things.
A new seal had the brothers trying to save reapers from Alistair. When their struggle ended, one reaper had died, the other one started to reap again, the angels had Alistair in custody and Sam held a dying Pamela Barnes in his arms. Her last words whispered in his ear: "I know what you did to that demon, Sam. I can feel what's inside of you. If you think you have good intentions, think again." Soon enough, Dean would know too what was inside him: Demon Blood. Sam was locked in Bobby's basement to detox, but he was freed by Castiel. He went after Lilith and killed her. Lilith was the last seal the only one needed to free Lucifer and Sam had believed Ruby's lies. The end was near, but for now, the brothers escaped on a plane, thanks to God.
It would be revealed to them that Sam was meant to be Lucifer's vessel and Dean was The Archangel Michael's. They were supposed to fight the battle of armageddon. Sam said yes and he was possessed by Lucifer. Their plan was to have Sam regain control for long enough to jump back into Lucifer's cage, which he did with the help of his brother. Castiel broke his body out of the Cage, however, his soul remained behind. Sam's soul was tortured for almost a year and a half while inside the cage. All the while his body pranced around without soul-making bad choices until his soulless body was reunited with Dean. Dean was really unhappy when he found out Sam had no soul.
Thanks to Dean Sam's soul was freed, but he had endured so much torture that a mental wall was built in his memories of his time in the cage. They hunted together, seeing dragons, Jefferson Starships, and other creatures until they faced Eve. Dean tricked her into biting him, the phoenix ashes inside of him killed her. Sam's mental block faded a little bit more every day, the threat of his memories coming back becoming more evident as time went on. It was shockingly revealed to them that Castiel and Crowley were working together to steals the souls from Purgatory. Amidst their attempts of stopping Cas. Castiel cornered Sam and broke his mental wall. Sam's managed to remain barely functional between his memories and constant hallucinations.
He was at least, conscious enough to witness Castiel's new power when he finally stole all the souls he could hold from Purgatory. He announced himself as the new god. When they finally convinced Castiel to return the souls, it was already too late. The Leviathans, ancient monsters, possessed Cas and they spread themselves in the water supply as Castiel's vessels disintegrated Leaving nothing behind, but a coat. The plan of the Leviathans was to take over. To feed on people and rule the earth. The brothers started to work to brought them down. Beheading and Borax did the trick, most of the time but they needed a permanent solution. While they were working Bobby was shot, they took him to the hospital, Bobby died in his hospital bed and the grief-stricken brothers mourned him.
Sam's grasp of reality kept slipping to the point he could not sleep or help Dean much. Dean located a resurrected and very much amnesic Castiel. When Castiel regained his memories he apologetically tried to rebuild Sam's wall, when he failed he took on Sam's mental distress within himself which made him lose his sanity. Dean and Sam stole a bunch of clay from Dick Roman. It took them some time to find a tablet inside the clay, that they couldn't read. With the help of a prophet, Kevin they discovered the tablet revealed a way to make a Leviathan killing weapon. A plan was devised, their aim Dick Roman, as Dick was stabbed with the powered bone he exploded unbeknown to Sam Dean and Castiel did not die, they were transported to Purgatory.
Dean was gone, Cas was gone, Bobby was pretty dead and the prospect of asking Crowley for help was utterly repulsive. No, Sam decided to quit hunting. He wandered without having a place to go until he fell in love with a woman, Amelia. Their relationship was good until her husband turned out to be alive. Sam left her to reconnect with him. He mostly kept an eye on cases around the area, he avoided getting involved as much as he could after all there were other hunters. It wasn’t until a case of a hungry kitsune attacking children pushed to get involved. It would be just one hunt, for the children. He ended getting hurt that how he would meet Emma Jameson. They quickly became friends.