Chainsaw Man has a major mystery at the moment, and that is the identity of the Fakesaw Man, who identifies himself as the Chainsaw Man. While Denji, the protagonist, is the real one, what this creature is is not fully explored, and there is a theory that he is a new kind of being in the series, which is a direct result of his contract with the Justice Devil.
There is an ongoing theory that Fakesaw Man made a contract with the Justice Devil so he could turn into his own version of justice, with Chainsaw Man being what he considers such, which makes sense with what author Tatsuki Fujimoto has established.
Therefore, he has turned into a flawed and monstrous version of the titular protagonist, boosted by justice and also by how feared Denji is in the story.
Disclaimer: This article is a speculative theory and reflects the writer's opinion.
Explaining why Fakesaw Man could be a new being in Chainsaw Man

Something that works as evidence for this theory is the fact that Fakesaw Man's appearance shifts from tall and muscular to a smaller and slimmer one, which could be a result of the contract. There is a chance that the person behind this identity made a deal with the Justice Devil and turned into his idea of justice, which was the titular protagonist, thus connecting his powers to him.
It could be revealed that author Tatsuki Fujimoto has connected this being's powers to Denji being feared, hence why the former's appearance changed from one arc to another.
The idea of the protagonist being feared and his powers fluctuating has been a major plot point in the second part of the manga, so this could be connected to this character.
Now that a lot of people have seen Pochita's Hero of Hell form, this could have boosted Fakesaw Man, which is why he made his move against the fake Death Devil and had the upper hand. This is a character who seems to be quite powerful, so there has to be a condition that allows him to reach that level.
The many mysteries of this character

The identity of Fakesaw Man has a chance of being another major twist in Chainsaw Man when considering the actions he has done in the second part of the manga and Fujimoto's record of surprising the audience.
In that regard, there is a chance that this character is going to be associated with some major plot point, especially considering his weird nature.
From a more thematic perspective, this creature could work as a reflection of Denji's initial desire in the second part to become a hero for fame and glory, with the element of the Justice Devil also serving in this metaphor if it proves to be the case.
Therefore, this could work as a way to challenge the protagonist's identity problems and what he wants to do with his life.
Final thoughts
The idea of Fakesaw Man being a new being who is connected to how feared Chainsaw Man is, while bizarre, is something that author Tatsuki Fujimoto would do in his series. Furthermore, it would give this creature a very peculiar role in the story, which would explain the focus he has received in recent times.
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