Eren Jaeger of Attack on Titan is a problematic protagonist—his emotions range from extreme compassion towards his friends to bizarre murderous rampages that take out entire cities. In fact, Eren's actions in the later arcs heavily parallel those of a stereotypical antagonist, considering that he pushes everyone away to remake the world as he sees fit.
All that aside, there is no question that Eren is a fighter, and has been so from the moment he met Mikasa Ackermann. Ever since the fall of Wall Maria, Eren has been involved in several skirmishes with Titans (many of them in the Attack Titan form). How does his warrior's journey begin?
10 Eren Saves Mikasa's Life For The First Time
Eren's first act of heroism—one of his only that isn't motivated by revenge—is saving Mikasa's life the first time they meet. He goes on a visit to the Ackermann family together with his father, who is stunned to discover a dead couple and a missing daughter.
Eren goes off on his own to find Mikasa's kidnappers despite Grisha's warning, and pretends to be a lost child so they lower their guard. Although he manages to finish off two men, he needs Mikasa's help to survive the last one. The pair become inseparable after this point.
9 He Constantly Protects Armin From Bullies
Armin's hope for a world tour sometime in the future is the only thing that makes him happy, but his exuberance is often misread as irreverence by other kids his age.
As such, Armin is the target for bullies, forcing his best friend to fight them off on his behalf. Eren proves how much he cares for Armin by attacking anyone who dares to hurt him in any way. And it's not a one-time event—nearly all the childhood fights Eren gets into are for the same reason.
8 Eren Defeats Jean Using Techniques Learned From Annie
Eren and Jean don't see eye-to-eye when they first encounter each other, but their brawl is stopped by a timely intervention from their friends. However, battling Annie teaches Eren some unorthodox new moves, which he later practices on Jean.
This time, Eren effortlessly takes his friend down, all the while delivering a sermon about Jean's lack of focus and insufficient training. The match takes all of three seconds; one of the shortest in the whole story.
7 He Saves Mikasa In His New Attack Titan Form
Eren sacrifices himself to protect Armin, instinctively activating the Attack Titan's first transformation. Powered by his anger, he begins destroying every Titan in the area.
Eren has no clue what he's doing in this state, at least on a conscious level, but it's interesting that he specifically chooses a Titan that's about to kill his beloved Mikasa. Although she doesn't instantly recognize him, she finds a certain familiarity in the Attack Titan's green eyes, indicating an unconscious bond between Eren and Mikasa.
6 Eren Fights Off A Horde Of Titans To Buy Time
Without knowing anything except that it kills only other Titans, the Scouts use Eren's Attack Titan to create a path for them to safety. Mikasa, Connie, and Armin lure him towards a group of Pure Titans so Eren can engage them and allow the Scouts to take refuge in a storage building nearby.
Eren's action buys enough time to save his friends' lives, but he collapses soon after killing the Pure Titan responsible for Thomas Wagner's death. It takes him some time and a lot of effort to convince everyone that he's as harmless as ever.
5 He Brings The Female Titan Down At Wall Sina
Eren's first battle against the Female Titan ends in a humiliating defeat, but the rematch concludes in a significantly different manner. As the Attack Titan, he attempts to trap Annie in Stohess District so the Scouts can interrogate her and presumably learn more about Titan transformations.
Regardless of his newfound determination, Annie's martial talents translate effectively into her Female Titan, and she pretty much dominates Eren once again. And yet, it is she who loses the fight in the end when the Attack Titan receives a curious power upgrade: lines of red lightning running down its face and body.
4 Eren Manages To Kill A Pure Titan Without His Powers
Eren travels with his team to check for any damage to Wall Rose, hoping to find out the truth of it's so-called breaching. He reaches Utgard Castle and jumps into the ensuing melee to rescue his friends from atop the tower.
This is the first and only instance of Eren actually killing a Titan without using his powers. Unfortunately, his excitement is cut short when his superior officer chides him, saying that he "was told to stay back." In the thrill of battle, Eren forgets that he is too valuable to put his life at any sort of risk.
3 He Manipulates Pure Titans Into Killing The Smiling Titan
Mikasa and Eren are on the run from Bertholdt and Reiner when they come across the Smiling Titan: Eren's mother's killer and, unbeknownst to him, his father's first wife. Hannes' death triggers something extremely deep and subliminal, allowing Eren to "command" the surrounding Pure Titans to do his bidding.
His roar brings them running to the area, and they pounce on the Smiling Titan and slowly eat it alive. Sadly, this massive victory does little to temper the fires of vengeance burning within him.
2 Eren Blasts Rod Reiss Into Pieces
Rod Reiss' gargantuan Pure Titan slithers towards Orvud, where the Scouts wait for it with an airtight, if somewhat risky, strategy. Eren psyches himself up as hard as he can before proceeding to lug a collection of gunpowder-filled casks into Rod Reiss' faceless mouth.
As a result, the Titan's upper body is blasted open; the explosion flings massive pieces of flesh in all directions, one of which contains Rod Reiss' human body. The latter's last rites are conducted by his daughter, Historia, but none of it would have been possible without Eren.
1 He Effectively Defeats The Armored Titan
Eren is initially awed by Reiner's seriousness and physical prowess, and often asks him for suggestions and advice regarding various matters. This is why the Armored Titan's revelation comes as a painful blow, and more so when Reiner kidnaps him and refuses to clearly explain the situation.
However, Eren gets payback for the betrayal by attacking and defeating the Armored Titan in a devastating battle that wrecks large sections of Shiganshina.