Attack on Titan author Hajime Isayama has a flair for creating characters with incredible depth, putting them through unimaginable scenarios where they experience serious loss and defeat. This of course leads to some incredible character development along the way.
However, protagonist Eren Yeager has a very specific type of character development. Most, if not all, shonen anime protagonists develop through the people they meet or the situations they have to find their way out of. Eren's character, alternatively, developed just from being in the world of Attack on Titan. Here's how he stands apart from the likes of Tanjiro and Deku.
Shonen Protagonists Are Often Influenced By The People Around Them
When looking at shonen protagonists, more often than not their character development is circumstantial. Take Demon Slayer's Tanjiro Kamado for example; his character progresses due to completely situational factors, such as meeting Inosuke and Zenitsu who push him to become a stronger demon slayer and vice versa.
In particular, Tanjiro's relationship with his sister Nezuko has presented the greatest development for the protagonist. After Nezuko was transformed into a demon, Tanjiro promised he would return her to her human form. Without the occurrence of this specific event, Tanjiro's character would never progress -- he would have either become an entirely different character or continued his life as usual.
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Another example of a shonen protagonist whose character developed through situational circumstances is My Hero Academia's Izuku Midoriya. In a society where Quirks are a status symbol, Midoriya's originally Quirkless personality dealt him his fair share of struggles. Life began to change when he was given the One For All Quirk by the #1 Pro Hero All Might. Again, had this event not happened, Midoriya would have led an unrecognizable life to the one fans have long followed.
Along with this, Midoriya's main drive to improve stemmed from his relationship with the hotheaded Katsuki Bakugo. Midoriya idolized his childhood best friend while Bakugo looked down on him, giving him the nickname "Deku". Midoriya pushed himself to become someone Bakugo could respect, training day and night to rank above him. Therefore, the protagonist's character development was caused by the events and relationships throughout MHA.
What Makes Attack on Titan's Eren Different From Other Shonen Leads
Eren Yeager is a character who has been broken down, forced to rely only on his core morals when faced with unimaginable horrors. The events of Attack on Titan are notoriously traumatic and Eren has had to witness all of them -- even the ones he didn't live through himself. He is a shonen lead who wants nothing more than to protect people he cares for in the name of love, but he was placed in a cruel world that only cares about survival.
If anything, Eren is not fit for the world of AoT; his character would thrive in any environment, but not this. His character development comes from realizing this, not friends or the countless traumatic events he's lived through. It's Eren watching his friends betray him, and seeing them die. It's his dreams being crushed before his very eyes. He's realized that it doesn't matter if you fight, or what you fight for because if it's not for survival then you can never win -- and truly be free.
By the end of everything Eren has been through -- even having to alter the past to trigger events and ensure he experiences just the right amount of horror to find himself in the right place -- Eren is tired and exhausted with reality. He went so far to try and balance survival with his goal to save his loved ones that he committed unspeakable atrocities, and this is what sparked his character development.
Eren was not broken by the Marleyan soldiers or even the Titans; his character development came from the greater world at large -- the savage world of Attack on Titan. His beliefs and needs were never in line with what was necessary to survive, and the crimes he committed to even attempt survival were irredeemable. Eren's fight was never really against Marley or the Titans, but rather a world he was never suited to live in. This is what truly sets Eren apart from other shonen protagonists: they belonged in their worlds, but Eren Yeager never did.