Chainsaw Man chapter 198 made a massive plot twist by revealing that Fami was the Death Devil all along, which puts her role in the story in a different light. Furthermore, there is an X user named @StarkyTheSalab who came up with the theory that Fumiko Mifune could be working in some shape or form with Death, thus explaining the former's actions in the series.
From the moment she was introduced in the second part of the Chainsaw Man manga, Fumiko has always appeared as someone who is hiding something. This has generated many theories over the years, but there is a chance that she could be working with the Death Devil for undisclosed reasons thus far, although that would explain some of the things she has done in the story.
Disclaimer: This is a speculative theory and contains spoilers for the series.
Explaining how and why Fumiko could be working with the Death Devil in Chainsaw Man
Now that it has been revealed that Fami is the actual Death Devil, it generates a lot more questions regarding the character of Fumiko Mifune and how she could be in cahoots with the powerful Devil. After all, she has been shown abandoning Nayuta and Denji when they were being attacked, and she is part of the team with Barem and Yoshida, so her interests seem to align with Death.
Furthermore, the manga establishes that Fumiko can keep herself from dying thanks to a clone because of a contract she has signed, which would make sense if she had done a potential deal with the Death Devil. There is also fan speculation that she can teleport, which is one of the fake Fami's abilities, although the story has yet to confirm this.
Fumiko has also claimed that she wants to keep herself alive, which could fit with the idea of working with Death herself. There is an argument to be made that she is scared of dying, and she decided to strike a deal with her to avoid it.
The character of Fumiko and the role of the Death Devil

A major reason the Chainsaw Man fandom believes that Fumiko is going to have a more significant role in the story is that she was introduced on her own in front of Denji and was given a decent amount of time with the protagonist. This tends to happen, especially with the female characters, when they have a role to play in the story, and Fumiko has yet to fulfill that role.
Furthermore, it was already established that the real Death Devil was pulling the strings of the fake Church along with Barem, so the ongoing mystery of her motivations remains in the series. Within that context, it is very likely that Fumiko wants to gain something from this, which could be a mere desire for survival or perhaps another ulterior motive that the author will reveal.
Final thoughts
It is likely that Fumiko Mifune has her own agenda in Chainsaw Man and could be connected to the Death Devil in some way. This could give both characters more fleshing out and add different motivations and ambitions to this current storyline.
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