The reason why this major character was introduced in You is actually pretty sad

For the last few weeks, it feels like everyone has been talking about Netflix's latest thriller, You. 

Based on the Caroline Kepnes book of the same name, the series  follows Joe Goldberg, a stalker-kidnapper-murderer/bookstore manager in New York City, who falls for a writer named Beck after a chance encounter. To try and win her affection, he stalks her - and kills anyone in her inner circle who he thinks will 'hurt' her or seemingly figure out what he's doing. The series was recently confirmed for a second season (and rumours of a possible season three), which has left fans trying to figure out where the story could go next. Especially after that cliffhanger, which is going to shake Joe "to his very core." But the wait for season two - as well as the general confusion over where the story may head - has left eager fans analysing every moment in the binge-worthy season one and comparing it to the book. Among all the biggest changes, though, is the introduction of Joe's neighbours. Yes, that's right: Paco, Joe's neighbour who he takes under his wing, doesn't exist in the book. Nor does his mum, Claudia, or her boyfriend, Ron. The trio live in the apartment next door, with the young boy often seen reading out in the hall when Claudia and Ron are fighting. Eventually, Joe forms a bond with Paco - sharing some of his food with him and bringing him books. The series' show runner, Sera Gamble, spoke to Variety about the very specific reason that Paco was introduced into the series - and how his introduction led to the "biggest tragedy" in the series. She said:

"Paco has been a character that illuminates the best, most compassionate side of Joe. “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 'thing' being, of course, when Paco sees Beck trying to escape from the basement in the bookstore and does nothing to help her. This is because, to him, he sees Joe as a good person after what he did to Ron. In making the choice not to help Beck, Paco proved his loyalty to Joe - which Gamble admitted she found to be "the biggest tragedy" in the finale. She said:

“Paco has been exposed to Joe [after] being trained to see Joe as this hero and saviour, do the biggest tragedy for me in the finale is that moment where Paco does the math and realizes there must be something very deeply wrong with her or Joe would never do something like this to [her]. “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.”

So, basically, Paco's character was introduced into You to show Joe's softer side - as well as how manipulative he could be.