Billie resurrected Jack and sent him to Earth to feed on Grigori hearts in an effort to make him stronger so that eventually he'll be able to fight Chuck.
Once the reunion is done, Jack reveals that Billie hid Jack in the Big Empty until God left this world, brought him back, and told him to eat Grigori hearts to start building his strength. It's all part of her plan to make sure he's strong to kill God.
According to Billie, Jack's status as a nephilim means he harbors the potential strength to take down God, but he'll need a considerable power-up to reach that level. This is why Jack is found eating the angel hearts - presumably to inherit the grace of his victims.
Jack tells the Winchesters about his hunting the Grigori and eating their hearts as well as the fact that he can't use his powers without God finding out about his resurrection and killing Jack again.
Jack Kline: Being the Nephilim son of Lucifer and far more powerful than Archangels, Jack's power was growing to the point where God feared that he would become more powerful than even himself to the point where he did not hesitate to admit this fear to Sam about his potential and his fear of Jack even trumped his ...
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.
Yet, Jack is just the child of a human and an angel, which would mean that Jack should be less powerful than Chuck. This is proved when Chuck easily kills Jack. However, Jack is brought back to life by Death (Billie), who implies that Jack has the power to harm Chuck.
While Lucifer and Michael were fighting Jack had been absorbing the energy that came off from them, including the energies released from the two archangels deaths. When he absorbed enough power he became unstoppable, immune to even God's attempts to smite him.
All angels, fallen or not, consider themselves family, each being brothers and sisters and referring to God as their Father. However, most angels have not actually met or spoken to God. According to Anna Milton, only four angels have actually met God and seen his true face.
Taking him for a spin in the Impala and showing him the basics of living life for a day are all sensible things Dean does to help Jack acclimate. Also, when Dean listens to Jack's soul-rejuvenating revelation of unintentionally killing Mary, he actually listens and forgives "the kid."
Jack Kline (b. May 18, 2017) is the son of Lucifer and Kelly Kline. He is the first and only known Nephilim to be sired by an Archangel. Jack was taken in by Sam and Dean Winchester who would teach him how to master his powers.
Some background: Jack, played by Alexander Calvert, is a Nephilim: a half-archangel, half-human hybrid with unprecedented sacred powers. His mother, Kathy Kline, became pregnant when she had an affair with the president of the United States while he was possessed by Lucifer, and subsequently died during childbirth.
After escaping from Hell, Belphegor possesses Jack Kline's corpse. At first, the Winchesters think that Jack has come back to life, but Castiel is able to tell that it's actually a demon using Jack as a vessel.
Jack returned to his mother's cabin and witnessed a hunter's funeral being performed on his mother and "father". Jack soon learned that Castiel was not his father but his uncle and that Dean's dislike in him comes mostly from the fact that Jack indirectly caused Castiel's death.
After going to Heaven, Dean is impressed and proud of the changes that Jack has made. Bobby Singer calls Jack Dean's kid while they talk and Dean doesn't say anything to the contrary, suggesting that Dean ultimately forgave Jack and came to see the young Nephilim as his family again.
In season 12 of Supernatural, a brand-new character was introduced who became one of the most powerful beings in existence. This was Jack, a Nephilim -- a baby born from the union of a human and an angel.
It is true that we believe that angels do not have gender. Angels unlike human beings are pure spirit. This is why when people say that a human being has become an angel in heaven that is incorrect. Any human being in heaven is a saint.
In the Kabbalah there are traditionally twelve archangels, who are each assigned to a certain sephira: Metatron, Raziel, Cassiel, Zadkiel, Camael, Michael, Uriel & Haniel, Raphael & Jophiel, Gabriel, and Sandalphon.
In the Old Testament, the book of Job, angels are referred to as “the sons of God” (Job 1:6). And when God is talking about the nation of Israel, he says: “When Israel was a child, I loved him, and out of Egypt I called my son” (Hos. 11:1).
Also, due to being spawned by the second oldest Archangel, Jack's power is so great that he has shown the ability to defeat and kill even archangels, as shown by how he was easily able to overpower and then kill Alternate Michael, who was connected to the Angel Tablet.
It is eventually revealed that Jack's soul really is gone since killing Michael and he has lost much of his morals as a result. Jack brutally killed Nick for trying to resurrect Lucifer, instead of doing it in a more painless way and in a moment of anger, lost control and accidentally killed Mary Winchester.
At the end, Jack coughs up blood in front of Dean before suddenly collapsing, bleeding from his nose and mouth. Rowena determines that Jack's condition is a result of his human and angel sides attacking each other without his grace maintaining a balance.
Chuck is God (and also he has a sister). In the show's 14th season, it threw another twist Chuck's way: Not only is he God, but he's been the author of Sam and Dean's lives, and when Sam (Jared Padalecki) and Dean (Jensen Ackles) decided not to follow through on Chuck's story, well, they pissed him off.
God wanted to end his Winchester story and write something new, but was becoming increasingly frustrated by Sam and Dean's staunch refusal to kill each other. Speaking with Amara in "Unity", Chuck reveals that his true reason for ending the world is frustration.
