Warning: Spoilers for Chainsaw Man episode 10 and Chainsaw Man chapter 39The Chainsaw Man anime just dropped what the manga later revealed to be Denji's greatest fear. Despite how horrific many of the series' devils are, none of them ever really get to Denji, which is why it's surprising that his only real fear is something so relatable.
In episode 10 of the anime, Aki crying over Himeno's death causes Denji to make a very unpleasant realization. He first realizes that he failed to do the same when she died - this despite the fact that Himeno was his first human friend in Chainsaw Man. And then he comes to the conclusion that this reaction would likely be no different if his other comrades met the same fate. Even the prospect of Makima dying convinces Denji that he would only be affected for no more than three days. This, combined with the fact that he felt sad when Pochita died before he became a devil himself, leads Denji to wonder if his heart is gone in more than one way.
Although Denji does seem to be concerned about his lack of humanity, he does shake off the feeling and acts as though he'd forgotten about it entirely. Later events in the manga though prove this isn't the case at all. In fact, there's a famous scene that the Chainsaw Man anime's opening song alludes to where Denji compares his reactions to other humans and is so worried by what he finds that he feels compelled to open up to Makima. For readers at the time, the fact that Makima reacted the same way as Denji was comforting since it proved that he'd not lost his humanity. But later events suggest that he may have actually lost his heart in more than one way. Part two of the manga hasn't had a chance to delve that deeply into Denji's psyche or state of mind since the focus has been more so on Asa Mitaka. It might actually explain why a recent Chainsaw Man chapter showed Denji tricking a homeless man without any remorse.
Denji Has Already Confronted This Fear in Chainsaw Man's Anime
Anime viewers have already seen Denji confront this fear again. Katana Man visibly appears to shake Denji to the core when he mentions that, as a fellow hybrid, even he is haunted by those he's killed when Denji indicated earlier that he doesn't experience this. Of course, Denji quickly shrugs this off, once more creating the illusion that he doesn't care when, in actuality, that isn't the case. Fans of the series can expect the anime's second season to delve much deeper into Denji's humanity, just as the manga has.
So while anime-only viewers won't discover this season that the prospect of him having his humanity is something that Denji truly fears, the first episode of Chainsaw Man season two will start off with a vengeance in this and many other capacities.