Shonen battle anime is not just a popular and defining genre of the medium, it’s a phenomenon, a formula. The shonen genre dates back more than a century in one form or another, but it wouldn’t be until series in the 1970s and 80s codified what we recognize today as shonen. Early hits like Devilman, Fist of the North Star, and especially Dragon Ball set the standard for what we now expect when we sit down to watch an action anime.

There have been minor tweaks through the years, including big shakeups coming with generational transitions, but the groundwork that was laid nearly 50 years ago is still adhered to today. There are outliers to this, of course, manga like One Piece and JoJo’s Bizarre Adventure have made names for themselves in not conforming to the norms of the genre, and they’ve both been running for decades. However, the best examples of modern shonen don’t attempt to reinvent what it means to be a battle series, but to refine that existing formula to a pure distillation of what fans love about it.

[minor spoilers for season one of Demon Slayer and the manga]

The best example in recent years of this trend is the smash hit Demon Slayer: Kimetsu no Yaiba, with a breakout in popularity after its climactic battle in episode 19 broke into the trending on social media, and the recent film Mugen Train broke box office records. Demon Slayer does not break the mold in many meaningful ways, and that’s purposeful; it’s not intended to revolutionize, but to polish the mold it was made in until it shines. Why strike out for completely new territory, when you can build upon the foundation that has already been laid?

First, a brief recap. Demon Slayer centers on Tanjiro Kamado, a teenager living in rural Japan in the 1920s, whose family is slaughtered by demons in his absence, and his only surviving sibling, his sister Nezuko, is transformed into a demon. After a run-in with the water hashira Giyu Tomioka, Tanjiro trains to become a demon slayer and discover a cure to Nezuko’s condition.

To clarify, when referring to shonen in this essay, I specifically mean battle shonen. This excludes action anime targeted at the shonen demographic that lacks most of the physical combat element (Death Note, Kaguya-Sama, Bakuman, etc.) Now, the things that make shonen so compelling are its generous use of tropes like the underdog protagonist who masters his fighting style through hard work and dedication, until a sufficiently powerful foe appears and renders that work meaningless. Then, when the chips are down, he pushes through through devotion to his friends and sheer grit. It’s generic, sure, but it’s a formula that has created many popular series.

There’s other elements that make an anime or manga shonen, as there’s usually a defined goal set from the first chapter or episode. Ed and Al want to restore their bodies, Naruto wants to become hokage, Deku wants to be the number one hero. These goals don’t just provide motivation for our protagonist, it gives the story forward momentum and contextualizes every encounter as progress towards that goal. It’s an unusually filler-resistant trope, considering the genre was at one point so laden with filler.

So how does Demon Slayer refine the tropes of the past? Well, the filler has a lot to do with it. Classics like Naruto and Bleach slowly grew more bloated over time as Shonen Jump pushed their authors to lengthen their run times, and the anime dithered while they waited for the manga to catch up. Demon Slayer solves this problem by twofold, first by the effectiveness of each of its arcs.

Demon Slayer’s arcs quickly fall into the formula of introducing a new hashira, or elite member of the Demon Slayer Corps who stands at the peak of their individual field of breathing. This way, when the time comes for the climactic battle with the lord of demons, Muzan Kibutsuji, we are familiar with almost every major player.

The second way that Demon Slayer avoids filler is by being a very short manga. While Bleach and Naruto both clock in at 700 chapters, Demon Slayer barely made it to 200 before retiring, and I cannot under-emphasize the importance of that. Gone are the days where a Shonen Jump manga will get stretched to its limits and broken, though this likely occurred as a cultural shift of the magazine after the unceremonious cancellation of Bleach in 2012. Demon Slayer is a short, compact narrative that ties up its loose ends without lingering on them.

How else does Demon Slayer perfect the long legacy of battle anime? It doesn’t hurt that studio Ufotable has done an absolutely amazing job on the mangaka Koyoharu Gotōge’s work, managing to capture the endearing style and visceral imagery without compromising their artistic vision. Similarly, it has a strong grasp of character design and a large cast so you are almost guaranteed to find a favorite among the wide variety of Demon Slayers. The breathing system is also an intriguing power, riding the line between a visual representation of the user’s fighting style and an actual elemental invocation. It demonstrates the user’s personality as well as the unique touches that they bring to their breathing.

There’s a good reason that shonen is the most popular genre of anime. Its well-worn tropes and hallmarks are familiar friends at this point to fans, and they’re universal enough to appeal to newcomers. The flashy style of fighting and characters are immediately eye-catching, and Demon Slayer is emblematic of how that formula works so well. I won’t deny that there is room for experimentation within the genre, but it is satisfying to watch these series build upon themselves in a race of one-upping each other. Irionically, it’s almost like they’re a macro-scale version of shonen rivals trying to surpass each other.