Jack Kline (Nephilim)

Lucifer's son is the unborn baby 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 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 render her powerless. 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".

Powers and Abilities
Lucifer's son is stated to have unimaginable power as the son of an archangel and a human. Even from the womb he displayed many powerful abilities.
 * Healing - From the womb, Lucifer's son was able to heal Kelly Kline's slit wrists and massive blood loss. While empowering Castiel, he also healed the angel's injuries from the fight with Dagon in a matter of seconds.
 * Resurrection - At the same time he healed his mother's injuries, Lucifer's son resurrected her.
 * Precognition - The child had a vision of the confrontation between Castiel and Dagon at Heaven's Portal which it was able to share with Kelly.
 * Power Granting - The child was able to temporarily empower the Seraphim Castiel while Castiel held his mother's hand so that they could take down Dagon together.
 * Super Strength - While empowering Castiel, Lucifer's son granted him the strength to effortlessly overpower the Prince of Hell Dagon.
 * Power Negation - While empowering Castiel, the child granted him the power to render Dagon completely powerless despite Dagon being empowered by Lucifer.
 * Pyrokinesis - While empowering Castiel, the child granted him the power to burn Dagon to ashes, a feat previously unseen with any other character.

Personality
Despite being unborn, Lucifer's son has 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, many characters believe he will naturally be born as an evil entity, too dangerous to leave alone. However, both his mother and Castiel who felt his essence no longer believe this, and both believe the baby will have a chance at becoming good.

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 possessing Castiel)

Trivia

 * Lucifer's child is the second known Nephilim in the series, the other being Jane.
 * The child is also the first known offspring of an Archangel.
 * It's indicated that the child will be more powerful than any other Nephilim due to being the child of an archangel.
 * 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 7 seasons later that the actual child of Lucifer would be conceived.
 * According to Sam, Nephilim don't require 9 month to birth, and Lucifer's child should birth on May 18th which corresponds to season 12's finale airing date.
 * When partially possessing Castiel and show Kelly Kline a premontion, their eyes briefly glowed gold.
 * Lucifer's son 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 Lucifer's son 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.