In Attack on Titan, no one is a hero, least of all Eren Jaeger. Though he starts out like a typical shounen protaganist, like Attack on Titan itself, it feels like the whole genre has changed now as much as Eren himself has. From the outset, he's enthusiastic about joining the Scout Regiment. Not just to fight titans, but to actually see the world beyond the Walls. Armin Arlert had something of an influence on him in that respect, from the time he showed Eren a book about the outside world. At the same time, Eren also shows a willingness to do whatever it takes to protect people, especially those he cares about. This is most darkly and forebodingly demonstrated when he rescued Mikasa Ackerman. He was willing to become a murderer, despite their both still being children, in order to save her from human traffickers.
But then the outermost Wall, Wall Maria, is broken through, and titans get inside for the first time, with Eren’s mother devoured right before his eyes. Understandably, he was more than a little upset. His enthusiasm to leave the Walls becomes superseded by a drive for vengeance that borders on psychotic. Jean Kierstein, who fills the role for the standard shounen rival, even calls him a suicidal maniac. Yet from how he handles discovering that his father, Grisha Jaeger, somehow gave him the power to turn into a titan, to being put on trial for being a titan-shifter, to being delivered into the custody of the Scouts, all where he’s had the opportunity to watch fellow comrades die in droves, the naivety of his motivation gets gradually chipped away.
Driven By Anger
His rage peaks even higher though when he learns that Annie Leonhart, Reiner Braun, and Bertholdt Hoover are not only also titan-shifters, but were, all this time, sent on a mission to kill everyone within the Walls. Yet it’s not long after that he suffers the blow of watching Hannes die because no matter how many times he tore at his hand he couldn’t seem to transform into a titan in time to save him.
He’s gritted his teeth and bit back cries before, but this is the first time things really seem to break him. He laments that he’s useless, that he’s still just a weak kid. In this moment of vulnerability, Mikasa reminds him why he means so much to her, and that seems to snap him out of it. Here he’s briefly gifted the power of The Coordinate—the power to control titans—and sends the other titans running off after Reiner and Bertholdt instead.
While this initially seems like a positive piece of growth from a dark moment, secrets buried within the depths of the Walls’ past start coming to light. Eren finds himself having to reconcile with the fact that not only did Grisha turn him into a titan-shifter more specifically by turning him into a pure titan and forcing Eren to eat him, but he also killed nearly the entire true royal family, even the children.
Yet for all of that, the rest of his friends in the 104th cadet corps acerbically bolster him to be what they need him to be, whatever he feels. He comes through, but he's also coming to terms with the idea that he was never anything so grand and special as humanity’s last hope. In fact, he learns later from Commandant Keith Shades, who harbored a secret crush on his mother in his youth, that his mother once said that Eren doesn't need to be special, that's he's special enough just for being born into this world.
Heroes Don't Exist
During the battle to take back Wall Maria, Armin comes up with a plan to take down Bertholdt as the Colossal Titan, resulting in Armin’s apparent death. They only have one syringe full of titan-shifter spinal fluid that they can use to save someone by turning them into the new Colossal Titan. Eren pleads with Captain Levi that Armin be given the injection over Commander Erwin, making the self-deprecating assertion that unlike himself, Armin cares about more than fighting. It’s subtle, but from where he started at the beginning, this is an appreciable moment of self-reflection on his own character.
After reading about his father’s past and learning of his relationship with Zeke Jaeger, as well as re-experiencing his father’s memories through their shared titan power, he’s sobered enough. But it’s the future that he’s sees when he touches Historia’s hand during the medal ceremony, her royal blood activating the Founder’s power inside of him, that shows him the path that lays before him. That’s when the biggest change in him yet appears to take place.
One four-year time-skip later, and Eren has become cold, calculating, and while still driven, also seemingly unconcerned anymore about how many lives he has to take, innocent or otherwise, to acheive his goals. Though he’s not unaffected exactly by Sasha’s death, he’s grown so narrowly focused on his goals that he's numb to everything else, and he reacts to it like a cruel joke rather than a tragedy.
Old Eren would have screamed a vow of vengeance as he did for Thomas at the beginning of the Battle for Trost. Although, as he tells Zeke later, his drive to do whatever it takes to protect his friends hasn't changed, only his approach to it. He's already realized that he himself is the one who pushes Grisha to murder the royal family, through the Attack Titan's ability to show future memories as well as past ones. With Attack on Titan the anime now drawing to a close, he seems to march forward not out of determination, but as though he already knows he's walking towards the inevitable end.