The relationship between Denji and Aki Hayakawa has been very turbulent so far in the Chainsaw Man anime. Denji's only goals are to kill demons, receive payment, eat, and get with Makima, their boss. This infuriated Hayakawa, who decided that his team in the Public Safety Unit of Japan wasn't the place for such a childish attitude.
In Episode 2, it caused a fight, and Hayakawa made the decision to use force to instill some maturity and sense in the immature teen. But Denji turned the tables, launching a vicious, gory, and horrifying attack on Hayakawa. Coincidentally, now that the guys have come to an understanding, this war strategy has evolved into a sinister tool of vengeance, enabling both to exact revenge on a nasty foe in the Season 1 finale.
Chainsaw Man's Denji Loves Nut-Kicking
Denji struck Hayakawa in the groin as payback for knocking him to the ground. Then, in a barrage of shots to Hayakawa's midsection, he made it abundantly clear that he doesn't follow the rules. Then, Denji would deliver the battered and bruised Hayakawa to Makima, blaming a Nut Devil for the assault. It was humorous, but it served as a stark warning to not undervalue Denji.
When Denji defeats the Samurai Sword in the Chainsaw Man Season 1 finale, this game is reinstated. He instructs Hayakawa to kick the opponent in the balls until he screams while still holding the opponent in his human form. The winner will be whoever yells the loudest and most painfully.
The guy's precious cojones are beaten in a punishing, squeamish scene that thankfully fades to an environment shot to lessen the pain — especially for male viewers. Hayakawa wants to take the high road but still engages. It is uncharacteristic of Hayakawa to decide to use police brutality on a criminal before the police show up; however, he does so in this case.
The Nut-Game of The Chainsaw Man Forms a Brotherhood
Now that Hayakawa was lamenting the terrorist organization that was Samurai Sword, Denji can tell that this rule-breaking, abusive session is more of a bonding one. They killed Himeno, which made Hayakawa depressed because he had no close relatives to think of. Denji exhorts him to exact revenge because the deed also destroyed his chance at a romantic relationship with her. It solidifies their bond as brothers, with Denji surprisingly displaying empathy, compassion, and care given that he is also an orphan in this world.
This is what the amusing Himeno would want, according to Hayakawa. He can now pay tribute to her eccentric legacy and make the Sword accountable for the deaths of the other agents. The fact that Season 1 ends with him, Denji, and Power shopping, living together, eating, and drinking shows Hayakawa that Denji is, in fact, a pseudo-sibling. In the end, they don't have much to rejoice about, but they can enjoy each other's company while they wait for information on finding Sword's employer, the Gun Devil, in Chainsaw Man Season 2.