The cowardly comic relief is an extremely old cliché in storytelling. Almost as old as the cliché of them actually being among the strongest characters in said story. You can find plenty of examples of characters like this in anime, from the action shounens to the slice of life comedies. They’re all generally similar; loud, annoying, played for gags, and far stronger than they should be.
Zenitsu is all of these things to an extreme. While he does have an extremely solid moral fiber, as we saw when he protected Nezuko from Inosuke simply because Tanjiro said the box’s contents were important, and he is a near unstoppable badass (at least when he’s asleep), he’s also a coward, a pervert, and a crybaby. A good 70% of his dialogue is delivered through terrified screaming and sobbing. All of which gets really old really quickly.
So why is it that so many people say he’s the most relatable character?
Part of that boils down to his cowardice. Because let’s be honest: if we were members of the Demon Slayer Corps, we wouldn’t be like Tanjiro or Inosuke or Rengoku or anyone else. We’d all likely shit ourselves at even the first sight of a demon. Don’t lie, you know you would. Those fuckers are scary looking, have magic powers, and are 100% determined to eat you. That shit is scary and Zenitsu doesn’t pretend otherwise.
That’s a large part of what makes Zenitsu an admirable character. Sure, he has to be asleep to actually be of use. But rarely does he actually run away from danger. We can see exactly how terrified he is practically 100% of the time. Yet when the chips are down, he’ll always be there to do what he can. Even if he thinks it isn’t much.
It certainly helps that his precious Nezuko is almost always right in the thick of it all, so he has a chance to at least try and look impressive.
But I don’t think that’s all there is to it. There are plenty of characters like Zenitsu and very few of them are as liked as he is. What else could there be?
Personally, I think a large part of it comes down to his abilities. See, every other character in the series, be they demon or demon slayer, have a multitude of skills. Whether it be demon magic or all the different sword forms. They’re all extraordinary in some way or another.
Zenitsu? He’s got one. One move. One trick. Of all the forms of lightning breathing, he can only use the first. He may have refined that to an insanely sharp edge, to the point where he’s actually stronger than any of the others in the main trio. It’s like that saying: fear not a man who has practiced a thousand punches once, fear a man who has practiced one punch a thousand times.
It might sound strange, but I find that more relatable than the massive bag of tricks. Most normal people have one skill that they spend their lives practicing. Whether that be art or medicine or a sport. Typically, they have a specialty. An artist might specialize in oil paintings or a doctor might focus their research on cancer. Seeing a character hone one specific skill feels more real than the huge variety of moves, off of which have been mastered.
Although Zenitsu definitely takes it to an insane degree. It is anime, after all.
It’s interesting to see how people have reacted to Zenitsu over the years. Characters like him are either the most popular (see Usopp from One Piece) or the least popular (see literally any other example of this archetype). And I look forward to seeing how people react in the coming season two.
But I doubt it’ll change much. Manga readers didn’t seem much to care.