|Dates||May 18, 2017 (born)|
|Location||North Cove, Washington|
|Episode(s)||12.23 All Along the Watchtower|
Jack Kline is the Nephilim son of Kelly Kline and Lucifer. He was conceived when Lucifer inhabited the body of President Jefferson Rooney, with whom Kelly was having a relationship. Angels believe that Nephilim are abominations that can destroy worlds; as Jack is the son of Lucifer, it is believed he will also be a force of great evil. However, after the Nephilim, while in utero, gives Castiel a glimpse of the future, Castiel believes Jack will bring peace and paradise to Earth.
Castiel senses a massive surge of celestial energy and discovers that a Nephilim has come into being. Sam, Dean, and Castiel realize that the Nephilim is the spawn of Lucifer. After abducting Kelly, they explain that Kelly is carrying Lucifer's child. At first, she doesn't believe them, but after she places her hand on a Bible, it bursts into flames. When Castiel takes Kelly to a diner after they exorcise Lucifer from President Jefferson Rooney, Kelly tricks Castiel into believing that she is going to the bathroom and escapes. She later calls Castiel and tells him that she can't get rid of her baby, stating that she is its mother and it's her child before hanging up.
Kelly, after experiencing some pain, demands that Dagon takes her to a doctor. He performs an ultrasound of the baby; he notices something unusual, but Dagon telepathically instructs the doctor to tell her that "everything looks a-okay." After Dean takes Kelly and tries to convince her that she can't have the baby, she tells them that she loves the child and refuses to get rid of it. When Dagon appears to take her away, she willingly goes with her. Later, Dagon chains Kelly to a bed and tells her that she cannot be trusted. Kelly tries to explain that she was worried about the baby, but Dagon bluntly tells Kelly that the baby will be just fine and that birthing a Nephilim is always fatal for the mother.
After spending a few days refusing to eat, drink, or bathe, Dagon forces Kelly to take her pills and tells her that her son will be born and bring an end to all no matter what happens to her, and that she should take a bath. Kelly, realizing what Dagon said may happen, attempts to kill herself, despite her love for her child. After she dies, the Nephilim revives her. Dagon eventually discovers what happened and tries to explain the baby saving her as an act to save himself and reminds Kelly that she will die when he is born. After this, Kelly is influenced by the powers of the Nephilim and believes that her son is not evil and will bring paradise to the world. After Joshua tracks a disturbance caused when the Nephilim revived Kelly, Castiel, Kelvin, and Hozai are able to track Dagon down. After Hozai is killed and Kelvin is captured, Castiel escapes with Kelly.
After exhausting the information they have on Nephilim, Sam is able to determine that it will be born on May 18. He realizes that since Nephilim are merely human souls infused with angelic grace, removing the grace could render the Nephilim human so that Kelly and her baby wouldn't have to die.
After communicating with Joshua, Castiel tries to bring Kelly to Heaven's Gate. Kelly tries to convince Castiel to let her baby be born, believing that it could lift the world to its feet rather than bring it to its knees. She explains how she tried to kill herself and how the baby was able to bring her back, but Castiel explains that her revival was not proof of goodness and he asks who will keep the baby on the righteous path when she is gone. Kelly asks him if he wants to feel the baby kick, but when Castiel puts his hand on her belly, he experiences a vision introduced by the Nephilim.
Sam and Dean are able to catch up to them and explain that they have a plan to render the baby human by removing its grace, but Kelly refuses to let them do it since she wants her baby to be born with all its powers. While waiting in the Impala for Sam and Dean, Kelly takes the keys and drives away from the motel, believing that Castiel will be able to take care of her baby when she is gone.
After arriving at the playground, Joshua is swiftly killed by Dagon. As she is about to kill Castiel, Kelly takes hold of his hand; through this connection the Nephilim transfers its power to Castiel temporarily and allows him to incinerate Dagon, killing her. Castiel, influenced by the Nephilim's power, decides to leave with Kelly to allow her child to be born. When Sam and Dean try to stop them, Cas knocks them out and they drive away in his truck. When Kelly asks what Castiel saw, he tells her the Nephilim showed him the future.
Dean talks to Sam about how the Nephilim was able to mess with Castiel's mind, despite not being born yet.
Castiel and Kelly prepare for the Nephilim to be born by renting a house in North Cove, Washington, purchasing a truckload of diapers, and preparing its room. Kelly even gives him a name: Jack. She makes a video for Jack to watch after she dies, telling him how much she loves him. When she feels a contraction, she unknowingly releases a bit of the Nephilim's energy, which makes a rift to an Alternate Universe.
While Sam, Dean, Castiel, and Crowley deal with Lucifer, Kelly finally gives birth to Jack in an explosion of light that renders Mary unconscious. When Sam notices the light coming from the home, he goes to check on Kelly, only to find her lying dead in her bed. Hearing a noise, Sam goes to investigate. He sees footprints scorched into the floor leading to Jack's nursery, where he finds an already adult-looking Jack huddled in the corner, smiling sinisterly as his eyes glow gold.
- He has been called Rosemary's Baby multiple times, in reference to him being the son of the Devil.