The following contains spoilers for Chainsaw Man Chapter 99 by Tatsuki Fujimoto, Amanda Haley and Sabrina Heep, now available in English through Viz Media.
Chapter 98 of Chainsaw Man concluded on an ambiguous note; it was unclear whether Asa Mitaka had turned into a hybrid where a human is fused with a Devil, made a contract with the War Devil or had turned into a fiend (when a Devil takes over a corpse's body). For all intents and purposes, it looked like fans had seen the last of the social recluse Asa and that Chainsaw Man would be embarking on a rather unusual arc of narrating the story through a Devil's eyes.
However, Chapter 99 has subverted those expectations. It opens with Asa waking up in her room, completely unscathed and her eyeballs still intact. Any sign of what had happened the previous night is completely gone, like it was a bad dream. Unfortunately it was very real -- and Asa will find herself much closer to the War Devil than she ever wanted.
Asa's situation in Chainsaw Man is unique. It doesn't really look like she's a fiend; even though the War Devil claims she died during the previous day's fight, Asa doesn't seem to be fully dead. At the same time, the War Devil had decided to leave half of her brain intact so she could still function and look like a human to the outside world when, in fact, the War Devil had claimed her body. If Asa doesn't do the Devil's bidding, she would really die.
These developments are quite from Denji and Pochita's relationship. They made a contract in which the former is in control of his body; the only time Pochita really appears is when Denji becomes Chainsaw Man. Pochita was moved by the young man's kindness and chose to sacrifice himself and become Denji's heart in exchange for Denji fulfilling his dreams.
The War Devil doesn't care about Asa at all though, and is using her as a way to complete her goal of annihilating the Chainsaw Man. Her supposed promise to return Asa's body if the two of them can defeat the Chainsaw Man is dubious. Even if readers were to assume the War Devil will keep to her word, there might not be much of a body left for Asa to return to.
In fact, what's happening with Asa looks very similar to Yuji Itadori and Sukuna in Jujutsu Kaisen. Just like the War Devil, Sukuna had forced Yuji into a pact by holding his heart hostage. There isn't an ounce of good inside Sukuna; the same goes for the War Devil in Chainsaw Man, who thrives on conflict and domination. The concept of good or evil doesn't exist in war. Unless the War Devil suddenly has a change of heart as she and her vessel embark on a school life, she is only concerned about destruction and doesn't care who gets caught in the crossfire.
One reason why the War Devil decided to merely claim Asa's body -- rather than take it over completely -- may be the nature of the Devil's powers, which lie in being able to turn what's hers into a weapon. In other words, claiming Asa's body will allow the Devil to weaponize her against Chainsaw Man.
The concept of claiming ownership isn't just physical -- it can be emotional too, as was seen in Chapter 98 when the War Devil was able to turn the class president and teacher into her weapons. That's why she tried to (unsuccessfully) ask out Yoshida, who unexpectedly reappeared in Chapter 99. By making him into her 'boyfriend', she could claim him as her own.
It's a chilling realization that this situation in Chainsaw Man is also a psychological war. Propaganda is weaponized during wartime by authority figures to emotionally manipulate people into enlisting. It's already clear that in this body, it's the War Devil who has control rather than Asa. Even Asa's thoughts aren't safe anymore. Any hope she has to regain control of her body is quickly being dashed.