Chainsaw Man has a major mystery that has never been fully explained, and that is why Pochita, considering his powerful status as the Hero of Hell, chose to give Denji, a random kid, a contract, thus building their connection. However, there is a chance that this could be connected to the protagonist's father, with the latter having had a contract with the powerful Devil before his son.
This would explain why, per the information in the Chainsaw Man manga, Pochita suddenly disappeared during the war in Hell since Denji took his father's life, thus severing the connection. It would provide greater context for Pochita choosing the protagonist suddenly and give author Tatsuki Fujimoto more information regarding Denji as a character.
Disclaimer: This article is a speculative theory and reflects the writer's opinion.
Explaining why Denji's father could have had a contract before him with Pochita in Chainsaw Man

It's shown in the first chapter of author Tatsuki Fujimoto's manga that Pochita first appeared before Denji during the latter's father's funeral. He is seen wounded, which is later revealed to be because of his battle in Hell, and the character doesn't want to interact with the protagonist, which is something that seems weird when considering his friendly nature.
This could be because Denji's father was the original man who had a hybrid deal with Pochita, hence why the Chainsaw entity was known as "man", not "Devil". Also, the Devil's initial rejection of the young boy seems out of character when remembering that chapter 179 shows his dislike of hurting children, so perhaps Denji killing his father was the reason why he was angry at first.
It would explain why Pochita and Denji met suddenly, especially during the latter's father's funeral, which seems far too convenient. Chapter 87 of Fujimoto's manga also explains that neither the Horsemen nor the Devils caused the wounds of the Hero of Hell, so this could be connected to Denji's father being murdered by his son.
What could this represent for the protagonist

The idea that Denji's father was the previous Chainsaw Man could provide greater context for Pochita's arrival in the protagonist's life and have more subtext to his backstory. It was already confirmed that he murdered his father, which is something that has traumatized him since childhood, so the fact that he is following in his footsteps could be another major blow to his psyche.
Moreover, there is a great chance to explore Pochita's character in isolation, who is someone who is very important to the story and doesn't get a lot of exposure in terms of his motivations and actions. Therefore, this could reveal a bit more of his characterization and what drives him, especially considering the recent conflict with the Horsemen in the manga.
Final thoughts
The revelation that Denji's father was the previous Chainsaw Man could be another twist by Tatsuki Fujimoto and something that could serve as another character study regarding the protagonist. Furthermore, it would connect the dots between Denji's first encounter with Pochita and how that has driven the plot forward.
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