The Chainsaw Man anime has only aired one season, but viewers have already become enamored with the characters. If asked who their favorite is, some people might say something like Makima, Power, or Aki. However, there may be some manga fans out there who say that their favorite character is Dennis.
Talking about Dennis may confuse some anime-only fans. As far as they know, there hasn't been anybody introduced in the series whose name is Dennis. In fact, it's hard to imagine where that name would come from; the whole anime has been set in Japan up to this point. Even if the manga fan gave in and admitted that they were talking about Denji, they could still be left tilting their heads in confusion. That is because the true meaning behind "Dennis" is an in-joke shared amongst hardcore Chainsaw Man fans.
Why Is Denji Called Dennis?
The joke can mostly be traced back to the Pochitaposting Facebook group. In addition to sharing memes and fan art for the series, the community likes to give joking names out to some of their favorite characters. Thus, they refer to Denji as Dennis.
Denji isn't the only one to get a joke name from the internet, either. In this same vein, Kobeni is known as Kenobi; this functions as an anagram for her name as well as a reference to Obi-Wan Kenobi of Star Wars fame. Aki also has a nickname, but it would be a massive spoiler for the anime-only fans. For now, those not in the know will have to be satisfied with Dennis and Kenobi.
As for how Denji, in particular, got the Dennis moniker, there's more concrete history behind that. Whenever people tried to type "Denji" on their phones in English, the name would be subjected to the whims of autocorrect; this is how "Denji" was unintentionally converted to "Dennis." Some people didn't realize this before posting their comments, so they ended up talking about Dennis when they meant to talk about Denji.
Those who caught on to what happened could have tried to avoid this mistake. However, they found it too funny and simply fed into it. Soon, everyone was calling Denji Dennis, whether they meant to or not. Fans should feel free to call the Chainsaw Man protagonist Denji or Dennis. There will come a time when the latter isn't just an in-joke among the die-hard fans, and it will be spread throughout the fandom. Of course, those who are in on the joke will still have to explain it to those who aren't.