Millennia ago the angel Gadreel let Lucifer into the Garden of Eden and was punished by being locked in Heaven's dungeon. While locked up, Gadreel became obsessed with reclaiming his good name and redeeming himself for his failure.
In 2013, the Scribe of God Metatron tricked Castiel into helping him gather the ingredients for a spell to make the angels Fall. Metatron succeeded in casting the spell with the unwitting assistance of Castiel and all of the angels in Heaven were cast down to Earth. Amongst these angels was Gadreel who replied to a prayer from Dean Winchester to save his brother's life from the damage he had sustained from the Trials of God. While helping, Gadreel posed as the angel Ezekiel who had died in the Fall and was said to be the opposite of what angels believed Gadreel himself to be. In order to save Sam's life, Gadreel was forced to take him as a vessel with Dean tricking his brother into giving the needed consent. However, to ensure that Sam didn't become aware of Gadreel's presence and expel him, Gadreel erased his memory of his near-death experience and he and Dean kept Gadreel's presence in Sam's body a secret.
When Sam and Dean start investigating a series of angel murders with Castiel, Gadreel is angered as Castiel's presence risks drawing the angels down upon them and he leaves the bar where Sam, Dean and Castiel are drinking. Outside, Gadreel encounters Metatron who recognizes who Gadreel really is despite his claim of being Ezekiel. Metatron asks why Gadreel chose to pose as Ezekiel and Gadreel tells him that its because Ezekiel is everything Gadreel himself is supposed to be not: a good and honorable angel though Gadreel insists the stories about him are not true. Metatron reminds Gadreel of how he was assigned to guard the Garden of Eden as he was God's most trusted angel yet he let Lucifer in. Gadreel insists that it wasn't his fault while Metatron tells him that whatever the case, humanity was corrupted by Lucifer and someone had to be blamed so Gadreel was locked up in Heaven's dungeon despite it breaking God's heart. Metatron tells Gadreel that he was the one who freed Gadreel from Heaven's dungeon when he made all the angels Fall, including the imprisoned ones. Gadreel is surprised to learn that Heaven is empty of angels except Metatron who tells him that while he liked it at first, he now finds his solitude to be tedious. Now Metatron wants to rebuild Heaven as God envisioned it with only handpicked angels living in it under Metatron's command. Metatron suggests that the way for Gadreel to gain redemption is for him to aid Metatron in his plans and leaves him to consider the idea.
The next day, Gadreel meets with Metatron again to further hear him out about his plans. The two angels agree on how chaotic humans are and Gadreel questions Metatron on what he intends to do, noting that as the new leader of Heaven, it would effectively make Metatron God. Metatron simply calls that semantics and tells Gadreel he'll go by "X" instead and insists that together, they can make Heaven a paradise again. Gadreel eventually returns to the Bunker to further consider Metatron's offer.
After Gadreel leaves to meet with Metatron again, Dean gets a call from Castiel about what he's learned about the coming angel war and that he is now an angel again after stealing the grace of another angel. Castiel reveals that he learned from Malachi that the real Ezekiel died in the Fall, worrying Dean as he realizes that Gadreel is actually an imposter. Realizing he needs to get Gadreel out of Sam, Dean approaches Kevin Tran to find a spell to "power down" Gadreel so that he can reveal his presence to Sam and Sam can expel him. Though Kevin questions Dean's request, Dean refuses to elaborate on why he wants to "power down" an angel.
At an overpass, Gadreel meets with Metatron again and informs Metatron he has decided to accept Metatron's offer to help him rebuild Heaven as Metatron's second-in-command. While Metatron is glad to have Gadreel's help, he tells Gadreel that he needs to prove his fidelity by killing their enemies who pose an imminent threat for him. While Gadreel is reluctant to commit murder, Metatron tells him its the only way Gadreel can work for him and gives him Kevin Tran as his first target.
Returning to the Bunker, Gadreel overhears a conversation between Kevin and Dean where they discuss the sigil they just painted that will hobble a possessing angel and realizes that Dean is onto him. To stop Dean's plan, Gadreel alters the sigil so that its ineffective and then poses as Sam, taking full control of Sam's body away from him. Posing successfully as Sam, Gadreel lets Dean lead him into the storeroom where he activates the sigil, not knowing that it has no effect on Gadreel. Gadreel impersonates Sam as Dean tells "Sam" the truth about what happened after the last Trial and how Gadreel is possessing him. After Dean tells "Sam" that he doesn't know who Gadreel really is and "Sam" has to expel him, Gadreel knocks him out and leaves the room.
Returning to the main room of the Bunker, Gadreel finds Kevin who asks "Sam" about Dean's odd behavior. Determined to complete the mission Metatron gave him, Gadreel smites Kevin, killing him while using telekinesis to keep Dean pinned to a pillar and out of the way. Gadreel tells him that there is no more Sam and he had only been pretending to be him after overhearing Dean's discussion with Kevin and altering the sigil. Declaring that he did what he had to do and that its for the best, Gadreel steals the angel and demon tablets and leaves, releasing Dean as he goes but still possessing Sam.
Soon after, Gadreel delivers the tablets to a pleased Metatron. However, Metatron is displeased that Gadreel didn't kill Dean. With the help of Crowley, Dean and Castiel capture Gadreel and help Sam force the angel from his body, freeing him. Gadreel then returns to his old vessel to continue his work for Metatron. Dean allowing Gadreel to possess him and Gadreel using his hands to murder Kevin causes great animosity from Sam towards Dean for some time starting after his rescue.
Following his murder, Kevin remains in the Bunker as a ghost though it takes awhile for him to be able to visibly manifest as Heaven is closed even to spirits. Kevin is eventually able to reach out to Sam and Dean who resuce his mother who he then leaves with. Two years later, God calls upon Kevin to convince the Winchesters to trust him and then finally sends his soul to Heaven where it belongs.
After receiving the stolen tablets, Metatron is able to tap into the power of the angel tablet to grant him God-like power while hiding the demon tablet in an abandoned library. With the power boost from the stolen angel tablet, Metatron is able to become nearly unstoppable.
After killing Kevin and stealing the tablets, Gadreel begins his work as Metatron's second-in-command by killing several other angels for him. Amongst these are Malachi when he refuses to join them. With Gadreel's help, Metatron is eventually able to gather a massive following, ultimately turning Castiel's own following against him with the use of a series of suicide bombings. However, this has the unintended effect of turning Gadreel against him as well and Gadreel aids the Winchesters and Castiel in finally defeating Metatron at the cost of his own life.
- Kevin's murder is one of the first times an angel is shown to be able to use smiting to kill something beyond a demon or a monster. The first appearance of an angel being able to use this power in this way was in the same episode when Bartholomew's Faction used the power to kill potential vessels for Malachi's angels during their battle with his faction.