While the ending of Supernatural sparked mixed feelings, the show, nonetheless, achieved television history as one of the longest-running sci-fi series in television history and one of the longest-running television series ever. With a particular focus on the supernatural, fantasy, and religion, the series introduces monsters, alternate worlds, and even has angels interacting with humans.
However, despite the genre, certain storylines push the envelope of realism and leave the viewer scratching their heads. There are many plot holes throughout the series and, depending on the writing team, the main cast goes through many "out of character" moments.
Realistic: Dean And Castiel's Slowburn Romance
Castiel's ionate "I love you" to Dean in the eighteenth episode of the final season was both validating and controversial. Some saw it as a harmful trope, while others saw it as a beautiful canon moment.
Dean and Castiel's relationship began as an uncertain allyship that, over a decade, bloomed into an unbreakable bond. Both the angel and the hunter experienced unspeakable trauma. Despite all that, they brought out the best in each other and accepted the other for all of their flaws.
Unrealistic: Adam As A Winchester
It has been a running gag within the fandom whether Adam, Sam, and Dean's stepbrother, would ever return at the end of season 5. As important a role as he ended up playing, it is questionable whether his storyline was necessary.
John Winchester may have been abusive and neglectful, but it is unclear how he would have been able to hide an entire second family from his wife and two sons; let alone the time to give Adam the childhood he withheld from Sam and Dean.
Realistic: Sam and Eileen's Romance
Throughout the series, it is clear that Sam has not had the best of luck with relationships. However, Sam's romance with Eileen Leahy, a deaf hunter, was not only realistic, it was important.
Ever since meeting her, Sam always saw Eileen as an equal and never treated her lesser than because she could not hear. Also, their relationship had an actual shot at lasing because she had experience with a hunter's life, meaning that they would know how to each other.
Unrealistic: Sam And Dean's Tension In Season 9
Season 9 was definitely one of the more emotional seasons of the series. Each brother faced death, the angels had fallen from Heaven and new chaos had come to Earth. The brother's relationship was once again put to the test, but not in a way that seemed right.
While Sam's anger towards Dean is justified (being possessed by an angel to stay alive), Sam's coldness towards Dean felt like overkill. This included telling Dean they could "no longer be brothers", that he wouldn't save him if he was dying, and generally refusing to work with him.
Realistic: Sam And Ruby's Turbulent Partnership
After Dean's horrific death at the end of season 3, it is understandable that Sam went through a deep depressive struggle. So his relationship/partnership with the demon, Ruby, is completely understandable. She found him at his most vulnerable and gave him a new path in life: exorcizing demons while saving the human.
Although it put incredible strain on his relationship with Dean, it is completely realistic that he would jump at the opportunity to use his supernatural powers for good.
Unrealistic: Lucifer Impregnating Kelly
Season 12 was a season full of changes: the Winchester's mother returns, a branch of hunters from across the pond is introduced and, most significantly, Lucifer conceives a child while possessing the President of the United States.
The idea that an archangel would impregnate a random human definitely pushes the envelope, even for Supernatural. This is made even more unrealistic given Lucifer's continued (and growing) contempt for humans. The silver lining is that the Nephilim, Jack, is a great addition to the series.
Realistic: Chuck Controlling The Winchesters
A prevalent theme throughout the entire series is the idea of free will and individual stories. At the end of season 13, it is revealed that Chuck Shurley (aka God) has been controlling the Winchesters since the beginning for his own amusement.
Chuck is obsessed with his characters and creations, and his goal is to create the most exciting stories possible, completely at the expense of said creations. Many of Chuck's comments and personality quirks of season 5 make a lot more sense after this revelation.
Unrealistic: Amara's Connection To Dean
In season 11, the series introduces Amara (aka The Darkness) as a foil to God. She was given the Mark of Cain by her brother to keep her in check. Through their mutual experiences with Mark, Amara and Dean are connected.
Although it can be argued that Dean has a "darker" streak due to his traumatic life, his connection to Amara feels shoehorned in and random. The season's plot would have still made sense with Amara being her own separate entity, with no connection to the Winchesters.
Realistic: Dick Roman's Rise To Power
It is safe to say that Season 7 was a transition season for the show: increased filler, multiple main character deaths, and the feeling of very low stakes for the season overall. However, the character of Dick Roman had a very realistic storyline.
The Leviathan leader went on a campaign to take over the food industry to turn all of humanity into sustenance. His charismatic personality and gung-ho attitude made him a media darling. His rise to power very much mirrors reality.
Unrealistic: Billie As God
Besides having, undoubtedly, one of the most iconic entrances in the series (if not television history), the character of Death represents the extremes of apathy and neutrality. In season 12, reaper Billie takes over as Death and, by the end of the series, she is scheming to become the new God.
As much as Death wants to restore order (especially in of what lives and dies) her gunning for the ultimate position of power seemed very much out of left field.