In the Shonen anime world, having a compelling character is incredibly important. These characters can make or break a series, and while much of that rests on the protagonist, in many cases the deuteragonist is just as important. Deuteragonists are like the secondary protagonists of a story, and while not all comics have them, they can be used in a multitude of ways to elevate the story and write compelling character arcs. In Shonen anime, countless exciting deuteragonists have appeared over the years, but only a few can claim to be the best.
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Tobio Kageyama
Tobio Kageyama is the deuteragonist from Haikyuu and one of the most important characters in the story. Kageyama is the equivalent of Hinata. While the latter excels in athleticism and physical ability, Tobio Kageyama excels when it comes to concentration and natural talent.
Kageyama is a setter and brashly overconfident all the time. His character arc sees him thrown out due to the overwhelmingly high standards he demands of his friends, only to finally come down to their level and form incredible friendships with them. Tobio Kageyama is just as important to the story as Hinata, and in some arcs even more so. The author does a brilliant job of fleshing him out as a real character.
Sasuke Uchiha
Sasuke Uchiha from Naruto series is as important as Naruto himself, as the author mentioned on many occasions. As Naruto grows in terms of strength and skill, so does Sasuke, and these two often act as Yin and Yang. The whole story about Naruto is based on these two characters, and they really continue to evolve in interesting ways.
As a deuteragonist, Sasuke goes through his own journey of self-discovery, which takes him from being a Konoha Shinobi to teaming up with Orochimaru and killing his brother, to even teaming up with the Akatsuki for a moment, eventually finding out what it really means to be a shinobi and then returning to the village after his battle with Naruto. Sasuke is truly an icon when it comes to anime deuteragonists.
Killua Zoldyck
Killua Zoldyk from Hunter x Hunter is one of the most intriguing characters in history. While Hunter x Hunter's the cast isn't the most varied and certainly not as tightly knit as in other shonen series, the characters themselves are incredibly well written. All four main characters in the story are compelling in their own ways, and Killua is arguably the best of the lot. In fact, Killua might just be the most important protagonist in the story, and as the deuteragonist to Gon, he does incredibly well.
Killua spends the time he has helping Gon, who he believes is his sun, in a variety of ways at the same time. He also has his own story to develop, such as his bond with his sister Alluka, along with other things. Killua's powers also made him an incredibly dominant personality. Whenever he is involved in an arc, he makes sure to leave a mark with his incredible abilities and never fails to impress the fans.
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Yuno
Black clover established its deuteragonist at the very beginning with Yunoand it goes without saying that he stands as a sharp contrast to Asta, the main character of the series. Yuno and Asta are very similar to Naruto and Sasuke in the sense that they compete with each other all the time. Both want to be the Wizard King, but at the end of the day, only one should be able to claim this title.
Yuno was a natural born genius right from the start, while Asta didn't even have the ability to become a true mage, let alone a wizard, let alone become the Wizard King. This dynamic of these two recognizing each other's abilities enriches both of their characters, and specifically for Yuno, it makes him one of the most compelling characters in the series and a brilliant deuteragonist.
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Vegeta
When Vegeta first appeared in Dragon Ballhe was certainly not planned to be a deuteragonist. However, as the story developed, his importance grew, and by the end of the series, he became the story's undisputed deuteragonist.
Vegeta and Goku have an iconic rivalry that has both going out of their way to outdo the other. Although Vegeta always lags behind Goku, it goes without saying that his character is more interesting to quite a few fans worldwide. Vegeta is a compelling deuteragonist and truly an intriguing character to watch.
Katsuki Bakugo
Katsuki Bakugo is one of the main characters in My Hero Academia and one that takes quite a lot of inspiration from the likes of Sasuke Uchiha. While Bakugo goes his own way, his relationship with Deku and the competition he enters with him is reminiscent of Naruto and Sasuke. Horikoshi has done brilliantly well to take inspiration from the best bits of the Naruto series and create a brilliant deuteragonist in Bakugo.
He's quite a complex character who goes through his own redemption arc, all while aiming for the same title as Deku to become the #1 hero. In doing so, Bakugo continues to achieve great things and even fights the final antagonist, impressing everyone in the process. Bakugo is also the most popular character in My Hero Academia, showing how successful he is as a deuteragonist.
Alphonse Elric
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Anime: Fullmetal Alchemist
Alphonse Elric from Full metal alchemist is the younger brother of Edward, who is the protagonist of the series. These two brothers embark on a grueling journey, and given that Alphonse gets the second closest attention in the series, it's clear that he's the deuteragonist here.
Alphonse loses his body at the beginning of the series due to trying to get his mother back. But the rest of the series is dedicated to him getting his body back, and it really adds intrigue to his personal character arc. Meanwhile, Alphonse doesn't hesitate to put his life on the line for his brother, despite dealing with his own issues, and that, combined with the immense strength and compassion he displays throughout the story, makes him a very compelling deuteragonist. Alphonse may be overshadowed by his brother Edward for most of the story, but he remains a beloved character that fans hold dear even over a decade later to this day.