So, given that Denji appears alongside the other heroes in chapter 259 of MHA, it's safe to assume that Chainsaw Man is seen as one of the many heroes on Earth-346.
Obviously, you might assume that it's merely a variant of Denji, considering that
Chainsaw Man chronologically takes place in the 1990s in an alternate universe where WW2 never happened due to Pochita consuming the World War 2 Devil (yes, really).
However, when you also take into consideration that Toxic Chainsaw (who All-Might fought five years before the events of DP Samurai and MHA) exists, it raises a question: how come there's both a hero and a villain who just happen to use Chainsaws? Well...
In CSM chapter 133, Fami mentions there being the existence of a Red Chainsaw Man (Denji in his Hybrid form) AND a Black Chainsaw Man (Pochita in his Devil form) to Asa & Yoru, with Asa wanting to save Denji by killing Black Chainsaw Man.
And to the average joe, while Red Chainsaw Man appears to be the heroic type that saves cats from trees, Black Chainsaw Man can appear more as a monster to be feared and run away from, which is fitting giving Pochita
IS the Chainsaw Devil after all.
Now, I can hear you all going "This is all interesting, Dredd, but what's the deal here?", and I'm getting to that. Now, we know for a fact that Hybrids in CSM are pretty much immortal and don't seem to age at all, as evidented by Whip Hybrid revealing herself to be 82 in chapter 143, in spite of her rather youthful look.
And considering that Denji would likely still be rather youthful if we assume that the Denji seen in MHA (2170s) is the same one that merged with Pochita in the 1990s, this brings me onto the main crux of the theory:
What if at some point, the Black Chainsaw Man side was split from Denji, and someone else happened to become the new host of Black Chainsaw Man?
((I'll continue this in a part 2 later. I have most of it written up, but the file limit is a slight hindrance.))
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