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The following contains spoilers for Chainsaw Man Chapter 104, "Spoiler", by Tatasuki Fujimoto, Amanda Haley and Sabrina Heep, available now in English from Viz Media.
Chapter 104 of Chainsaw Man was significant in terms of plot development with the two heavy hitters, Chainsaw Man and War Devil meeting for the first time, but it was also important in a more subtle way. Just before Denji and Asa meet at school, the chapter focuses on Yoru and Asa. The two of them talk, and Yoru reinforces her hatred of Chainsaw Man even though Asa is more neutral towards him now as he had inadvertently saved her life.
Near the end of the conversation, Yoru admits something to Asa, consequently shifting previous perceptions of her. All of a sudden, Yoru ironically feels more human. Despite being sworn enemies -- the War Devil shares some surprising similarities with Denji.
Infuriated that she and Asa had gotten saved by her mortal enemy, Yoru does the only logical thing she can think of: she punches a pillow and throws a tantrum. Coming from the War Devil whose mission is to bring back nuclear weapons in order to start back the war, it's unexpectedly endearing to see such a human reaction, albeit immature. Despite being part of the Four Horseman and being in the same league as Makima, Yoru doesn't have the same imposing aura as the Control Devil.
Asa asks Yoru why she hates Chainsaw Man, and the War Devil tells her that the two of them had fought a long time ago, with Chainsaw Man having "eaten from" her body, weakening her. As explained in Part 1 of Chainsaw Man, the reason why the majority of Devils fear Chainsaw Man is because he has the ability to consume Devils and erase them from existence. Although the concept of war has not been erased from people's memories, many people have already forgotten about great wars in the past, much like when Makima mentioned World War II and Kishibe had no idea what she was talking about. In an unexpectedly vulnerable moment, Yoru tells Asa that she's afraid that people will forget her before falling asleep.
Although Yoru is set on defeating Chainsaw Man so that he will "vomit nuclear weapons back up," her admission shows that, ultimately, she doesn't want to be forgotten. Her sense of self is strongly linked to her identity as the War Devil, and if she were to be completely consumed by Chainsaw Man, she would cease to exist. Although Devils are never completely dead since they go through this endless cycle of reincarnating in Hell; once they reincarnate, their memories from their previous lives are completely gone. This is what happened when Denji ate Makima: the Control Devil reincarnated into a young girl named Nayuta who doesn't remember her past life as Makima.
In some ways, Yoru's goal is the same as Denji's. Ever since he returned in Part 2 of Chainsaw Man, he has been valiantly trying to tell the world he's Chainsaw Man. Although he claims that he's doing this because "the ladies will be all over me," the reason is much deeper than that. Right now, the world is making him out to be a hero even though, ironically, Chainsaw Man was never meant to become a hero since he's supposed to be an embodiment of people's fear of chainsaws. Denji, by all accounts, is nothing like what a hero should be: he's raunchy, brash, and simple. However, as Chainsaw Man, he's loved by the citizens. Denji's sense of self-worth is heavily dependent on his identity as Chainsaw Man.
This is also why it initially feels as though Denji hasn't changed since Part 1 -- in some ways, he can't. He fell in love with Makima, but all she cared about was Pochita, or the Chainsaw Devil. Makima told him that, as Denji, he essentially had no value. Since then, Denji hasn't been told otherwise and still bears the heavy weight of his unresolved trauma. He likely still believes that he can only be loved if he's Chainsaw Man. While Yoru is intent on fighting Chainsaw Man to preserve her identity as the War Devil, Denji is more than happy to ditch his human identity and wholly embrace Chainsaw Man.