It turns out one of the key characters in Netflix's YOU wasn't even in the book.

Warning – This post contains mild spoilers for Netflix series YOU.

It may have been almost a month since YOU premiered on Netflix, but fans are still deep diving into theories about the psychological thriller.

From fan theories about what will take place in the second season to pointing out the similarities and differences between the series and Caroline Kepnes’ novel, it’s safe to say we’ll be hearing about YOU for quite some time.

But while there’s a number of differences between the novel and the Netflix series, one change in particular is bewildering viewers.

You see, one of the key characters in YOU, Paco, never appeared in the 2014 novel.

In case you’ve forgotten, Paco is Joe Goldberg’s vulnerable young neighbour who sparks a friendship with Joe while dealing with a mother who struggles with drug addiction and her abusive partner.

Throughout the series, Joe lends Paco books and protects him from his mother’s abusive boyfriend, leaving viewers with conflicted feelings about the stalker-turned-serial-killer.

But while you might be wondering why Netflix decided to introduce a completely new character, it turns out Paco was there for a very specific reason – to show Joe's compassionate, charming side.

"Paco has been a character that illuminates the best, most compassionate side of Joe," showrunner Sera Gamble told Variety.

"He educates Paco, he gives Paco a sandwich when he's hungry, he keeps Paco safe. And then [Paco] does this incredible thing in the finale [in return]."

The introduction of Paco didn't just bring out a more human side of Joe, it also showed how easily people are manipulated by him.

"In choosing not to help Beck, Paco proved his loyalty to Joe. Paco has been exposed to Joe [after] being trained to see Joe as this hero and saviour, so the biggest tragedy is that moment where Paco [thinks] there must be something very deeply wrong with her or Joe would never do something like this to [her]," Gamble explained.

"His faith in Joe and his trust in Joe was so deep that Beck could literally be screaming through that door, but what Paco has just learned is that if Joe is doing something like this, he must have an incredibly important reason."

Haven't watched YOU on Netflix yet? Check out the trailer below.

Gamble also told Digital Spy that Paco was created to give viewers a greater understanding of Joe's childhood.

"You can tell a lot about Joe from the way he treats Paco, but also here's a young person who's looking for guidance and so it organically puts Joe in a situation that he's sharing his life philosophy in a way that's instructive to us about what he's doing," Gamble told the publication.

According to Gamble, season two of the thriller will delve into Joe's childhood and his relationship with his mentor Mr Mooney.

"Joe had a pretty terrible childhood and that does contribute to who he is as an adult," she said.

"But we also want to make it clear that a lot of us had shitty childhoods, and most of us don't choose to do what Joe Goldberg does."

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