Adam Milligan

Adam Milligan was born September 29, 1990 to Kate Milligan and John Winchester, and therefore is the half-brother of Sam and Dean Winchester. John did not mention anything about the "supernatural" to Adam in order to protect him. He was an Eagle Scout, graduated from high school with honors, and attended the University of Wisconsin, where he majored in biology and was pre-med. He is currently the vessel of Michael the Archangel.

Season 4
In 1990, John killed a ghoul. Years later, seeking vengeance, the ghoul's children killed Kate and Adam Milligan and assumed their forms, hoping to lure John into a trap. "Adam's" call reaches Dean and Sam instead, as John died two years earlier. Dean and Sam discover the trick, kill the two ghouls, and burn the body of the brother they had never known. Dean says he was their brother and died like a hunter, so he deserved the funeral of one.

Season 5
Later, sensing something has happened, Castiel goes to his grave, and after defeating some angels he finds a revived Adam and takes him back to Bobby's. He tells them all that the while he was in heaven, the angels had told him that he was destined to become Michael's vessel and that he was willing to do it. Despite branding him with Enochian sigils to keep him out of the angel's sights and keeping him captive in the house, Zachariah contacts Adam in his dreams and he helps Adam escape. Upon meeting Zachariah in the Beautiful Room, Adam learns that angels brought him back to life in an elaborate scheme to get Dean to say yes to Michael. Upon arriving at the Beautiful Room - which is revealed to be an enclosed set built inside of an abandoned muffler factory in Van Nuys, California - Dean agrees to be Michael's vessel after Zachariah begins to torture Sam and Adam. However, after Zachariah summons Michael, it is revealed to be a ruse and Dean stabs him with an angelic blade. After Dean kills Zachariah, the room starts to shake with the approach of Michael, so Sam, Dean and Adam prepare to leave. Unfortunately, Adam gets trapped inside the room and the Winchesters are unable to get him out. He later returns in "Swan Song" as the vessel of Michael facing off against Lucifer in Kansas. When Lucifer tries to convince Michael to join him and rebel against Heaven, Michael refuses and prepares to fight his brother until Dean loudly enters the cemetary with the Impala. Before either Archangel can act, Castiel uses a molotov cocktail on Michael filled with holy oil, banishing him from the cemetary for a time. As Sam regains control of his body and is about to jump into Lucifer's Cage, Michael tries to stop him and both end up falling in. Due to Sam's release from Hell Adam's own status is unknown, but it is presumed he and Michael are both still trapped in the Cage.

Season 4

 * "Jump the Shark" (Not really him)

Season 5

 * "Point of No Return"
 * "Swan Song" (As Michael's Vessel)