Hajime Isayama's Attack on Titan is one of the most popular anime of the last few years and Eren Yeager, the poster-child for the story, is its central protagonist. Though the show has many other important characters, Eren is still considered to be the lead, dominating both the manga and the anime's plot. But is this how it should be?
Armin Arlert, Eren's childhood friend and one of the story's other central characters, actually embodies the role of the protagonist much better than Eren. Eren is merely the audience's viewpoint into the world of the story, whereas Armin is the character that actually pulls the plot forward.
A protagonist must carry the weight of being more than a story's main character -- they must be self-motivated, active and embody the overall theme of the story. Armin is the character that satisfies all of these requirements. Whereas Eren is motivated by plot events such as the death of his mother and the disappearance of his father, Armin is motivated for internal reasons. When Eren and Mikasa join the military they are surprised when Armin decides to join as well, considering he is physically weaker than most other candidates. But Armin tells them about his dream of seeing the ocean. This dream is what motivates him forward, despite the death of his own parents.
Armin is also much more active in the storyline. During the Battle of Trost District arc, Armin is the one who makes the plan for baiting the Attack Titan in order to be captured. Similarly, it was Armin's idea to use Eren in Titan form to plug up the hole in the wall. Armin is known to be one of the most intelligent characters of the show and it's very evident when one starts to look into the story.
Armin always has a pivotal role. It was Armin who first connected Annie to the Female Titan. It was also Armin who suggested looking in the walls for Reiner, as he knew that the walls had hollows where people could hide. Eren merely acted on Armin's plans -- if Eren was the true protagonist these would be his plans, or he'd at least have a few of his own.
Another factor that makes Armin a truer protagonist is the fact that he's always supporting and motivating the other central characters, especially Eren. When Eren is discovered to be the Attack Titan, Armin defends him to their fellow soldiers, explaining that Eren has more value in battle as a Titan than as a human. Eren's life is spared because of that infamous speech. There are other moments when Eren doesn't have his own motivation to go on, too, like during their journey back to Shiganshina. Armin reminds Eren of their dream to see the ocean and the outside world and this makes Eren feel good enough to keep going.
Ultimately it's a joint effort between all the characters to propel the story forward, but Armin Arlert takes on one of the largest roles. Armin definitely does more as a protagonist than Eren and he does it in subtle ways that don't draw focus away from the main story. A pivotal character in Attack on Titan, Armin is one of the main points on the chain of causality that allows the storyline to continue.
