The Winchesters and Adam

Dean and Sam are brothers while Adam is their half-brother whom John kept away from the hunting lifestyle. So unlike Sam and Dean, Adam was not a hunter. Despite this, Adam shares some personality traits with his half brothers, especially Dean. The two share their similar sense of bravery and desire to protect their family, as well as their sarcasm, looks, and opinion on women. It also appeared that the two think that family is more important than blood. Because of this, the youngest and oldest share a more Sam and John like relationship, the latter pair also have similar personality traits. Sam and Adam's relationship was a more Sam and Dean like relationship, with Sam sharing a beer with Adam and telling him that with his attitude, he'd fit right in around here. They both have a shared resentment for their father. All three of these brothers are descendants of Cain and Abel.

Background
Adam was born September 29, 1990 to his mother Kate Milligan. When he was 12, he begged his mother to call John. When John heard he had a son, he dropped everything (Sam and Dean remember that and didn't know what happened until they met Adam). He was an Eagle Scout, graduated from high school with honors, and attended the University of Wisconsin, where he majored in biology and was a pre-med. Then a week before "Jump the Shark", Adam was eaten and killed by ghouls. The ghoul now impersonating Adam called John, but gets Sam and Dean instead.

Season 4
(For now we're going to refer to the ghoul being Adam, as Adam. When its revealed that "Adam" is not Adam, there will be quotation marks). "This thing killed my mom. If you're hunting it, I want in."

- Adam

In "Jump the Shark", Sam and Dean are very surprised to hear they have a half-brother. Dean, is very suspicious, and thinks it to be a trap. So they head to a diner to meet him. Dean steps up the area in case this "Adam" is a shapeshifter, or demon. Sam finds out that Adam is actually a real person through his research and confirms it further with clues from John's journal. Then Adam shows up, and Dean begins to ask him questions.

Frustrated, Dean tells Adam, that he's not John's kid, he and Sam are. Adam is surprised that he has brothers. To further prove that Adam is their brother, he shows them a photo of him and John. So they help him figure what happened to Adam's mother and they figure out she was grabbed by something. Later Adam comes to their motel room demanding to know what was really going on. Sam is the one (despite Dean's protests) who tells Adam about hunting. Adam seems cool with it and wants it. Dean says No. He also points out that John wanted to protect this kid (which is why he ripped the pages out). When Dean leaves Sam teaches Adam anyways.

As Sam is teaching Adam, he hears something in the vents and get Adam out of there. They head to Adam's truck, but Sam is grabbed by something from underneath. Adam rushes to help Sam, and then Dean arrives on the scene and helps. Sam suggests they use Adam as bait (much as he dislikes the idea. Dean outright refuses) and starts to train him. Sam and Adam have a brotherly bonding moment. Dean is still not liking the idea because he wants Adam no where near the hunting lifestyle.

While Dean is out, Sam and Adam get the place ready for the creature. Then Adam "mother" walks through the door, and Adam rushes to her. Sam tries to convince Adam to step away from his "mother", but "Adam" knows this and knocks Sam out. Meanwhile, Dean discovers Adam's body in the crypt. Adam's "mother" does reveal that Adam really was their brother.

Season 5
"Adam, you may not believe it, but Dad was trying to protect you, keeping you from all this." "Yeah, well I guess the monster that ate me didn't get that memo."

- Dean and Adam

In "Point of No Return", Dean, Sam and Adam are re-united again when Zachariah brings Adam back from the dead. But Adam isn't what they met a year ago (it was the ghoul pretending to be him). Sam and Dean tell Adam that they are his brothers. Adam doesn't really care about that. He demands to know where Zachariah is. He also tells the boys he is Michael's vessel, which surprises Dean. This motivates Dean even more to say yes now. Sam tries to reason with Adam using John, Adam but retorts that John was the guy who took him to a baseball game once a year; nothing more. Sam responds that they need time and they have a plan. Adam asks what the plan is, and Dean remarks on the power of love being the plan. Sam just begs Adam for more time.

In a dream Zachariah tricks Adam into telling him where Adam is. He tells Adam that Sam and Dean care about each other more than Adam. When Adam goes missing, Sam and Dean go after him. They find Adam (who realizes he was tricked) and he tries to warn Dean, but it's too late. So Zachariah starts to torture Sam and Adam so Dean will say yes. Dean caves and tells Zachariah to call Michael. But then Dean notices Sam's disappointment and kills Zachariah. They all attempt to escape the room before Michael comes. Dean goes to help Sam through and Adam follows behind. Unfortunately, the door closes before Adam can get out. Dean tries to help Adam, but he can't open the door. When he does, the room and Adam are gone.

In "Swan Song", Dean finds Michael in possession of Adam's body. He tries to speak to his half-brother, but Michael brushes him off. When Sam is about to jump into the Cage, Michael tries to stop him, but gets pulled in along with Sam. This leaves Dean to watch as both of his brothers goes to Hell.

Season 6
In "Appointment In Samarra", Dean tries to make a deal with Death to get Sam and Adam's souls out of Lucifer's Cage. Dean tells Death that he'll return his ring to him in exchange for their souls, but Death reminds Dean that he was only supposed to be borrowing his ring in the first place, and anyways, he knows where Dean hid it. He doesn't completely reject Dean's request, however; he tells Dean that he has to make a choice between Sam or Adam's soul. Dean chooses Sam.

Season 10
In "Fan Fiction", Sam and Dean visit a play based on the Supernatural books. Among the cast is a student dressed as Adam. Sam and Dean initially don't recognize who this is, until Maeve points it out. This leaves an uneasy feeling in the brothers.