Jack Kline is an important character in Supernatural, but he’s only been around for three seasons. He first appeared in the Season 13 premiere, 'Lost and Found', in 2017. As the only Nephilim ever born to an archangel, Jack came with a lot of lofty expectations from the beginning. Being the son of Lucifer projected him as inherently evil. But he was born innocent… as a teenager with immense and erratic powers.
He imprinted on Sam, Dean, and Castiel as his parents, and from there, his awkwardness and issues make serious trouble for everyone else. His insecurity and growing powers, especially when he made mistakes, created situations that caused Sam and Dean to fall into conflict with God himself, lose everything they worked for and loved, and left them alone as he got worse and worse.
Existential Crisis At Birth
Jack is born into a teenager’s body with rudimentary English language skills and the mind of a clueless child. His powers are dangerous and unknown and, while he’s off trying to figure himself out, Sam and Dean are distracted from more important things. There's a rip in their reality opened to an apocalyptic universe, their mother is stuck there with Lucifer, Castiel is dead, and everyone in Heaven and Hell wants Jack's power for themselves. Jack is interested in the wonders of nougat and finding himself and his family, no matter who his powers hurt along the way.
His Daddy Issues Caused Major Universal Destruction Across Worlds
He first looked for Castiel, and his insistence in finding Castiel led to a catastrophic chain of events. The Shadow from The Empty is let loose to wreak havoc amongst Heaven's angels and even affects events on earth.
Sam and Dean give Jack a home, but he’s still insecure in many ways. His urge to please them and search for validation leads him to Lucifer, alternate universe Michael, and the literal wrath of God. Lucifer and Michael's plans kill many hunters they all love, the gruesome bargain Castiel struck with The Shadow, and eventually, Chuck/God kills Jack in his complete frustration.
He Developed An Affinity for “Monsters”
During his searching for who he was and who he would be, Dean’s aversion to Jack led him to wonder if he was like one of the monsters they hunted. This caused his affinity for others called “monsters.” He wanted to believe so badly that those that were born from monsters could be good, considering he was born from Lucifer.
When Lucifer found Jack, he used this to his advantage and tricked Jack into believing he could be good too. The series of events resulting from this gave Michael an opening out of Apocalypse World, to wreak havoc in the main universe, and to eventually force Jace to burn off his soul to kill him.
His Will Woke The Shadow From The Empty
Though innocent at first, Jack’s power is still the force that woke The Shadow in The Empty and gave Season 14 and Season 15 the most formidable enemy that Sam and Dean had ever faced. Because of this awakening, The Shadow tricked people, manipulated situations, and ultimately even took Billie (Death herself) off the playing field. The Shadow also resurrected Lucifer at the end of Season 15 to try and give the Winchesters a real weapon that might kill Chuck, and that plan went completely sideways.
He Endangered Bobby’s Colony in Apocalypse World
When Jack first arrived at Apocalypse World, he spent time trapped with Michael and knew his wrath. Even after that experience, he wanted to be loved. He wanted to be the hero. He wanted so badly to be good that he stayed in the Colony and put everyone there in danger. His desire to do something worthwhile brought Michael and other angels down on them. He helped to fight them but it was a battle they didn't need to be involved in had it not been for Jack's incessant desire for validation and redemption. Even in another universe, he was worse.
Soulless Wandering Jack Hungered For Power
As his soul burned off, his insecurities left him and the desire to be good was replaced with the desire to be powerful. When that power took over his morals and Mary Winchester questioned his actions, his will killed her. Because of him, Sam and Dean lost their mother again and Jack completely went off the deep end. They didn't know what to do with him or what he might do with the world. He became the biggest threat to everything they loved.
Manipulated By Duma
Without his soul and hungry for power, the angel Duma (who had taken control of Heaven) manipulated Jack to become the “new god.” After Mary’s death, Jack was lost and looking for his place with the Winchesters once again. He still wanted more power and a reason to use the considerable power he had. As the “new god” he punished people brutally in accordance with the Bible’s Old Testament. He tried to make more angels but just ended up killing a lot of humans that didn’t deserve to die. As a soulless God, Jack was worse than ever.
Threw The World Into Honest Chaos
After his stint as the new God failed miserably, Jack was imprisoned and then escaped from the Ma’lak box meant to keep him trapped forever. In his distress, he put the whole world under a spell so that everyone told only the truth, throwing all of Earth into chaos and inviting the wrath of Chuck.
People found out about what really goes bump in the night and the Winchesters were hindered from doing their job. Jack was a failed God but he was even worse as a jilted kid who was sick of being lied to by everyone he knew.
His Soul Returned, Jack Plans A Dangerous Sacrifice
Chuck provokes Sam and Dean to kill Jack but they won’t do it and he kills Jack himself. He dies without a soul and is sent to The Empty. Billie resurrects him and he wanders the earth soulless with a plan to make a big sacrifice in order for Billie to put things to rights. He visited the Garden of Eden through the Occultum to get his soul back and Billie’s plan hinges on his newfound urge for redemption. What Jack doesn’t realize is that Billie’s plan will put everyone back where they should have been, including Eilleen, Charlie, Bobby, and everyone else that came back from the alternate apocalypse universe or was resurrected. Because he is steadfast in this plan, Chuck’s wrath is further provoked and everyone they love is taken from them, including Cas invoked his curse so that The Empty would take Billie.
In The End, Jack Became A “Hands-Off” Deity
Jack became a power vacuum and his plan with Michael, Sam, and Dean led to a resurrected Lucifer trying to kill Chuck himself. Jack sucked up all their power and he was the one to kill Chuck, and take all his power too. He was more powerful than God and Amara together. He put the whole world to rights but didn’t bring back all the people that had died because of his plan. Sam and Dean have to go on alone while Jack goes off to be a deity in some “hands-off” manner he never really explains. He becomes the most powerful being in the universe and doesn’t do a thing to make up to Sam and Dean all the grief he caused them. This final non-act isolated Sam and Dean for the rest of their lives and made Jack the absolute worst.