The following contains spoilers from Chainsaw Man Part 1 and 2 by Tatsuki Fujimoto, available now in English through Viz Media's Shonen Jump imprint, as well as Season 1 of the anime, now streaming on Crunchyroll.
Chainsaw Man chapter 111 introduced a new threat to the series' characters, giving Denji and Asa a new problem to deal with. After Yuko made a contract with the Justice Devil, the high school girl was transformed into a Devil -- meaning she had to flee from society altogether. After Asa and Yuko said their final goodbyes, promising to see each other again, the latter met an interesting fate.
As she was running away over the rooftops, Yuko was stopped by a figure who resembled Chainsaw Man -- despite Denji being shown sleeping just moments prior. The Chainsaw imposter quickly decapitated Yuko and disappeared, which left fans around the world questioning who this fiend could be. One theory suggests the second Chainsaw Man could be the high schooler's upperclassman, and Denji's mentor, Hirofumi Yoshida.
Who Exactly is Yoshida?
Yoshida was first introduced in the "International Assassins" arc of Chainsaw Man Part 1, and has piqued interest from fans ever since. The devil hunter doesn't work for Public Safety, so Makima had to privately hire him to protect Denji from any threats, which explains why the character didn't stick around after the group's battle against the assassins. However, in this short time, it was clear that Yoshida takes his job seriously and has the same ruthless approach as Public Safety.
Following the end of Chainsaw Man Part 1, Yoshida reentered the plot in the "Justice Devil" arc. As a high school transfer student, he ran into Denji at school, admitting that he was there on a mission from the mysterious organization he works for. The upperclassman is at the school to stop Denji from revealing his identity and told him that if he tried this he wouldn't hesitate to intervene. Eventually, Yoshida was forced to grab Denji with the power of the Octopus Devil before he revealed himself, demonstrating his willingness to keep his word. In short, Yoshida is around to protect Denji from his own recklessness.
Yoshida Has a Contract With the Octopus Devil
The main grounds for this theory relate to the devil Yoshida is working alongside. The devil hunter has a contract with the Octopus Devil -- and some even question whether he himself is the devil. Octopuses are known for being able to change their bodies to camouflage with their surroundings, so many fans believe that Yoshida is using the Octopus Devil's transformational abilities to act as the second Chainsaw Man.
So far, it appears as though Yoshida has control of octopus tentacles when he summons the help of this Devil, so the theory suggests that Yoshida either makes a tentacle mirror the image of Chainsaw Man or uses the power directly to distort himself into the imposter. While he may have the means to pull this off by using the Octopus Devil, what motive would Yoshida have to do this?
Yoshida May Be Protecting His Mission
Yoshida may be somehow posing as another Chainsaw Man to protect Denji's identity. He was tasked with this by a very mysterious organization, so no one knows what could happen to him if he were to fail. It's possible that the outcome could be violent, or even fatal, so Yoshida is even more dedicated to ensuring Denji doesn't declare himself as Chainsaw Man. Additionally, because there is no known information about this organization, they could well have resources for Yoshida to dip into -- which could also help him make an imposter.
If a second version of the fiend is around while Denji is asleep or in school, his rock-solid alibi would throw anyone else off his scent. Even if Denji were to declare himself as Chainsaw Man if he was somewhere miles away when another was seen no one would believe his claims -- especially as a 16-year-old schoolboy. By using the Octopus Devil to pose as Chainsaw Man, Yoshida would protect his job and keep Denji's alias undercover.
Denji is purposefully trying to be recognized as Chainsaw Man for female attention, and it's his upperclassman's job to make sure this doesn't happen. So, even though this is only a theory, Yoshida is one of the few characters with both the means and the motive to pose as Chainsaw Man.