Jack is the son of Lucifer and a human woman named Kelly Kline. He is a Nephilim in the CW's hit-tv series Supernatural.
While unborn Jack resurrects his mother after her attempted suicide, this had caused Kelly to see he was not evil.
During Castiel and Kelly's talk, Kelly explain to Castiel of this and he is skeptical since Jack could of done so to prolong his life. Kelly then has Castiel feel the baby kick as he is amazed by this. It was at this moment, Jack briefly took control of his mother and chose the angel to be his protector, since he felt safe with him. Jack gave Kelly a glimpse of the future and had her head to the Heavenly Portal where Dagon attacked.
When Dagon overpowered Castiel, he approached Kelly and told her to flee but the latter refused and grabbed his hand as Dagon moved to kill the angel. Jack then took control of his mother once more and empowered Castiel, allowing the latter to heal himself and kill Dagon by incinerating her. This also won him Castiel's faith and the latter wanted to ensure his birth after seeing the future.
At the end of All Along the Watchtower, the newly born Jack aged himself into a young adult smiles at Sam with his eyes glowing yellow.
During Lost & Found, Jack meets Sam and asks if he is his father but Sam states he is not. He knocked the Winchesters out and wandered around to find his father before he wandered through the forest and was taken to jail for help. He met Sam again and they talked with Jack stating he aged himself into a young adult as his mother told him while he was in the womb that it can be dangerous for a baby. Jack asks about his father and says Lucifer wasn't him but Castiel was since his mother told him that's who will protect him. Jack asks about him and is told Castiel is dead, to his dismay.
In Patience, Jack overheard the argument between Sam and Dean about him with Sam defending him and Dean blaming him for Castiel's death. After this, Jack sadly said Castiel's name, with his eyes glowing in the process. Jack's voice telepathically reached The Empty where the essence of Castiel was sleeping before he awakes hearing Jack calling his name.
In The Big Empty, Sam offered Jack the chance to help in a case and mostly does grunt work for Dean. He learns more about hunting through the brothers and soon learns they are dealing with a shape-shifter. He watches as the brothers confront a good shifter and learns there is another one causing trouble. Jack has the good shifter Mia help him get closure for his mother's passing and instinctively uses his powers to save Sam from the evil shifter Buddy who was planning to kill everyone, which allows Sam to kill the monster. Jack comforts Mia by stating she helped him in his pain as she took his words with a smile. Jack is soon given approval from Dean and takes it with a smile.
In Advanced Thanatology, Jack was mentioned by Sam to have stayed at the Bunker as he and Dean worked on a case. Dean mentions him to Billie, the new Death about how the rift opened.
In Tombstone, Jack was on the computer doing research before the brothers returned, he asks how the case went as they act strange and he wonders what's up as they revealed they brought a surprise. It is a resurrected Castiel who greets Jack. Shocked, Jack questioned this as he learned that what gets burned stays dead and he is asked if he had something to do with it, as he revealed he begged for Castiel to return which woke him in the empty. Castiel thanked him as Jack was relieved to have him back and quickly hugged his uncle. Jack then revealed he was researching and showed the group a case, which he suspected was zombies. While Dean humored him they decided to go through with it as Jack caught up with Castiel about his not being there for him and that Jack's mother was right about him being good. Jack's research about the case proved correct but it turned out the monster was a ghoul. After finding the monster, Jack tried to help Dean but ended up killing an innocent bystander and fell into a depression. Castiel and Sam tried to comfort him but Jack angrily told them to stop. After Dean killed the monster, Jack felt guilty for his actions as everyone tried to comfort him but he left the group after apologizing for flinging them around.
In War of the Worlds, Jack hid his presence from everyone so he wouldn't be tracked which proved useful as the angels were trying to find him to replenish their number and Asmodeus was tracking him. Once Asmodeus captured Castiel and Lucifer (who returned from the other world), he realized he'll need Jack to help him if an alternate version of Michael invaded.
In the The Bad Place, Jack decided to master his powers and wanted to prove he was good and searched for other realms to find Mary Winchester for her sons as he knew they wanted her back. Jack's search led him to find Dreamwalkers, people who can see other worlds and after meeting one, he found the world Mary was trapped in. After finding a more stronger dreamwalker named Kaia, he failed in convincing her to join before he was found by Sam and Dean. He told them of his plans before letting them see the vision of the world Mary was trapped in. Mary was shown in a cage being tortured by spikes stabbing her repeatedly as she called out for help. He later revealed to the Winchesters, he was not trying to find Lucifer who he doesn't see as his father or family and that them as well as Castiel are his only family. This comment was met with agreement by Dean who praised him for his actions to save his and Sam's mother, as Jack smiled at these words. After the angels kidnapped Kaia, they ask Jack to join them and be with his own kind but Jack denied they were his family since they killed innocents and helped the Winchesters in killing them. Dean then forced Kaia (who learned of Jack's family) to join them, Jack apologized to her and they bonded when he showed her a peaceful world. They soon combined their powers to create a rift, but as a result of Kaia's lack of focus and the angels' attack. Everyone was separated, with Jack being the only one sent to the world that Mary was trapped in as she was confused by his presence and identity while he knew who she was.
In Devil's Bargain, upon finding out that Michael is in the Apocalypse world, the Winchesters and Castiel decide to go back to that world and save Jack as well as Mary from danger. They enlist Donatello Redfield to use prophet abilities to open a rift after reading a angel tablet. Asmodeus also discovers Jack is in the other universe after disguising himself as Castiel and getting information from Donatello.
In Good Intentions, Jack was revealed to have been captured by Michael and his subordinate Zachariah plays scenarios in his mind to get him to open a rift. Jack is trapped in a series of scenarios in attempt to get him to use his powers but he sees through them and meets Michael who subdues him and throws him in a cell with Mary. He talks with her and reveals his identity as a Nephilim. He tells her that her sons sent him and they aren't present in Apocalypse World and she tells Jack to let Michael kill her since they can't allow him to invade Earth but Jack wanted to rescue her. They escaped when they learn the fortress has a spot of weak warding and the two meet the Alternate Bobby and go to his colony. Jack entertains survivors but uses his powers which Bobby takes note of and is angry when Mary tells Bobby what he is. Bobby ordered for him to be exiled out of distrust and Mary decides to go with him. The next day, angels invade and Bobby blames Jack since it was him who drew them to him and he tells Jack he can't help. While running Jack hears screams of survivors and remembers what Sam and Dean taught him, he turns back and finds Mary being confronted by Zachariah who overpowers her and demands Jack's location. Jack calls out to Zachariah and uses his powers to immobilize him before he kills the angel in front of Mary and Bobby who just arrived. Bobby alerted Jack of three more angels who were coming and he killed them in mid-air. Afterwards, Bobby thanked Jack who decided to use the teaching Sam and Dean taught him before he decided to truly stop the angels he has to kill Michael, which shocks mary and bobby.
In Bring em' Back Alive it was mentioned by Charlie Bradbury that Jack and Mary have formed a resistance against Michael. In the main universe, Gabriel (Jack's uncle) is informed of his nephew who was sent to the other world.
During Funeralia, Jack sees many victories over Michael's forces but Mary is concerned about the survivors they are taking in. Jack has grown closer to her as she now sees him as part of the family and he sees the same in her.
In Beat the Devil, Team Free Will and Gabriel capture Lucifer to open a rift to Apocalypse World as they set off to find Jack and Mary. In the main universe, Lucifer is informed by Rowena that Jack was sent to Apocalypse World and he breaks free of his binds to kill the witch but she repels him, accidentally sending him through the rift. Jack reunites with Dean and Castiel before he meets his uncle Gabriel. He is informed of Sam's fate and becomes distraught, as Castiel tries to comfort him but he ignores him and questions both his uncles on them not resurrecting Sam as they tell him they couldn't as they aren't strong enough. Jack then watches Sam arrive alive and smiles at him, before he sees that he is being followed by Lucifer who greets his surprised son.
In Exodus, Jack is shocked to meet his real father but Dean stops him from trying to speak with him. Jack watches an argument between everyone break out and teleports away as Gabriel goes to find him. Alone, Jack ponders on his thoughts as he recalls his mother telling him nobody would blame him for looking for his father. Jack returns to camp where he decides to hear Lucifer out, to the latter's delight. Jack is told of his other family such as his grandfather God and his uncle Michael, as Jack expressed interest in God and remarks while he didn't meet his real uncle he met the latter's counterpart who tortured him. Dean tried to stop the conversation but Jack stops him since Lucifer is his father. The conversation continues and Lucifer soon points him out to his uncle Gabriel who Jack states he already met and they talk with him, as Jack hears his uncle criticize Lucifer and warns him not to trust him since Lucifer justifies heinous acts by playing the victim. He evacuated with the camp and spoke with his father, he watched in amazement as his father destroyed a squad of angels on the road. He believed with Lucifer, Gabriel, and himself that he could beat Michael. Jack later took part in the rescue of Arthur Ketch and Alternate Charlie, he killed several angels. As everyone prepared to evacuate through the rift, Jack decided to kill Michael before he left though Sam tried to talk him out of it. Lucifer stepped in and warned Jack that it would be a wasted effort, seeing his point Jack complied and evacuated. Jack was one of the first to pass through, returning to Earth. He was later sad at Lucifer's departure as well as being told he was killed by Michael.
During Let the Good Times Roll, Jack joined the Winchesters and Castiel in hunting werewolves and they are successful after Jack immobilized them with his telekinesis.
As seen in Stranger in a Strange Land, Jack is learning hand-to-hand combat by Bobby in order to make up for his lack of powers. Sam tries to comfort him but fails. During the rescue of Castiel, Jack is captured by demons and tries to fight them off but is beaten badly before watching Sam threatening the demons. Castiel speaks to Jack and tries to get through to him by comforting him on how his power will recharge in time. Jack is not moved by the latter's words and is angry since Michael is loose on the world while he can't stop him.
In Byzantium, Jack dies from his illness and ascends to Heaven. He is able to perceive his time there wasn't real while with Castiel and the Winchesters. Leaving his personal heaven, Jack is chased by a black ooze but eludes it and heads to the heaven of his mother. He interrupts her childhood memory and reveals his status as her son to the latter, as she changes to her adult self. Jack embraces his mother and reveals her fate, while she accepts it but is saddened to realize he's dead as well.
During Ouroboros, Jack exorcises Michael from Rowena's body and finally kills Michael by incinerating him and absorbing his grace. Jack reveals Michael is dead and tells them his powers have returned and proves it by showing his angel wings.