Jack Kline (Nephilim)

Jack is the son of Lucifer and Kelly Kline.

Background
After Lucifer took possession of President Jefferson Rooney, he had sex with Jefferson's aide and lover, Kelly, who was unaware of the possession. After their first round, Kelly said she believed Jefferson Rooney (Lucifer) would make a great father, which inspired Lucifer to sleep with her again and conceive a child.

The conception of the Nephilim was felt by Castiel, who was miles away inside the Men of Letters Bunker. He and the Winchester brothers decided they needed to get rid of the child, as it was the "spawn of Lucifer".

After Sam and Dean successfully re-imprisoned Lucifer, Castiel took Kelly away to a diner. He pleaded with her to get rid of the child, informing her that it was not Jefferson's, but the Devil's. Kelly excused herself to the bathroom and fled the diner. As she rode away in a taxi, she insisted through a phone call that the baby was still hers, and she did not want to get rid of it.

After his mother was rescued by Dagon from angels trying to kill her and him, Dagon revealed to his mother Kelly that he was a boy. Elsewhere, Lucifer mentioned to Crowley that his son's power is a chip off the old block. When Kelly began experiencing "weird little pains", she ordered Dagon to take her to a doctor, in case something was wrong with the baby. Dr. Turner could not find anything wrong with the baby itself, and Dagon ensured Kelly that the child was fine and he was going to become big and strong. The pains were in fact the start of intense, life-threatening pain that Kelly was going to experience, since birthing a Nephilim was always fatal to the human.

In The Future, Kelly only has less than a month left before she gives birth and is told by Dagon of the unimaginable horror the child will unleash upon his birth. In an effort to stop this, Kelly commits suicide only to have her child resurrect her. Following her resurrection, Kelly states that she felt his soul surge through her and that it was good, not evil though Dagon believes the child only brought her back to ensure his own survival.

After being kidnapped by Castiel, Kelly insists on the goodness of her child. Castiel is ordered by Joshua to bring Kelly through the portal to Heaven which will kill both Kelly and her child and scatter their atoms across the universe. As Kelly continues to insist on her son's goodness, Castiel asks her about if this is true, who will guide and protect the child after his birth since Kelly will die in the process. The child kicks moments later and Castiel feels it. As the Winchesters arrive, the child has a premonition of a conflict between Dagon and Castiel at Heaven's Portal and instructs Kelly to take herself and Castiel there, having chosen Castiel to protect and guide him after sensing Castiel when he touched Kelly's belly.

At Heaven's Portal, Dagon kills Joshua and effortlessly defeats Kelly's defenders. As Castiel and Kelly stand hand-in-hand, the child empowers Castiel, causing his eyes to glow. With the child aiding him, Castiel is easily able to overpower a shocked Dagon and negates her powers. Telling Dagon to "call it a miracle", Castiel uses the child's power to incinerate the Prince of Hell.

Following Dagon's death, Castiel tells the Winchesters that it was both himself and Lucifer's child who killed Dagon. Castiel tells the Winchesters, who planned to remove the child's grace and leave him human and powerless, that he now knows that the child must be born with all of his power intact as he now has faith in the child's goodness. As they depart in his truck, Castiel tells Kelly that the child showed him "the future".

Castiel takes Kelly to a lakeside house which is set for a newborn baby. Kelly is desperate for her son to be born good and well-loved. During labor, a portal to another dimension is opened and Team Free Will use it to trap Lucifer within but he drew Mary with him as they tried to push him in. Sam returns to the house and finds Kelly dead. He also finds a trail of scorched foot prints leading to the nursery, and discovers a fully-grown Nephilim with golden eyes sitting in the corner, creepily smiling at him.

Personality
While still in his mother's womb, Jack was shown to be aware of his surroundings. He glanced at Dr. Turner during an examination and actively chose Castiel as his new protector and guide, replacing Dagon and even going so far as to help Castiel kill Dagon.

Since his father is Lucifer, it is a general assumption that Jack is the embodiment of pure evil. This belief was originally shared even by his mother, who attempted suicide so that her child wouldn't be unleashed upon the world. Jack resurrected her, however, saving himself in the process. Kelly then became convinced that Jack could be good, and that the world needed him. Eventually, Castiel also became hopeful that Jack could bring peace. He claimed that he was shown a vision of a future without pain, fear, or suffering.

Upon delivery, the baby rapidly ages and leaves his dead mother on her bed while he retreats to his nursery. When Sam discovers him, Jack is now an adult and smiles menacingly at the hunter.

Physical Appearance
Upon birth, Jack took on the form of a human adult. Unlike any other being seen on the show, Jack's eyes glow gold. Like his father, however, only his irises glow.

When Sam discovered Jack hiding in the corner of the room, he was visibly frightened by Jack's somewhat creepyhhh appearance.

Appearances

 * Season 12
 * LOTUS  (mentioned only)
 * Stuck in the Middle (With You) (mentioned only)
 * Family Feud (mentioned only)
 * Somewhere Between Heaven and Hell (mentioned only)
 * The British Invasion (mentioned only)
 * The Future (mentioned and briefly empowering Castiel)
 * Twigs & Twine & Tasha Banes (mentioned only)
 * All Along the Watchtower
 * Season 13

Trivia

 * Jack's name is a medieval diminutive of John. Jack was used as a slang meaning "man". John, on the other hand, means "Jehovah has been gracious".
 * He is the second known Nephilim in the series, the other being Jane.
 * Coincidentally, both of them have a 4 letter name that starts with J.
 * He is also the first known offspring of an Archangel.
 * He is the first and only known being to possess golden eyes.
 * Sam had a misconception that the Antichrist was the child of Lucifer because of what he learned from the lore, but Castiel corrected that misunderstanding by revealing that Jesse Turner who was a half demon hybrid was the actual Antichrist in I Believe the Children Are Our Future. It wasn't until seven seasons later that the actual child of Lucifer would be conceived.
 * According to Sam, Nephilim don't require nine months to be born, and he was born on May 18th which corresponds to All Along the Watchtower, Season 12's finale.
 * When empowering Castiel and showing Kelly Kline a premontion, their eyes briefly glowed gold.
 * He bears similarities to The Source's Heir from Charmed. Both are the unborn child of the show's version of the Devil, both are able to channel their powers from the womb and both play a role in the deaths of a powerful, apparently unstoppable demon, Dagon in the case of and The Tall Man in the case of the Source's Heir. Both demons were also killed by a powerful fire ability that incinerated them.