Armin Arlert was one of Attack on Titan's main characters. When he was first introduced, he was one of the weakest characters in the manga, but was also incredibly smart. Throughout the series, he accomplished a lot, whether it be due to the enemies he defeated or the plans he came up with in order to save those he cared about.
By the end of the manga, Armin had achieved more than most of the other protagonists and antagonists. These are just a handful of his greatest accomplishments, including ones that have yet to be animated.
10 Graduating From The 104th Cadet Corps
One of Armin's biggest goals, since he was a child, was to join the Survey Corps so he could travel outside Paradis' walls. However, he had to work hard in order to make that dream come true by training in the 104th Cadet Corps. Despite having a tough time doing some of the things that were required of him to graduate, he was able to do so and became a Survey Corps soldier.
9 Saving Jean's Life
When the Survey Corps fought against the Military Police, Jean was nearly killed. However, Armin shot the MP who almost killed him in time, saving Jean's life. While it was great that Armin rescued his friend, he felt terrible about having to kill a person, especially since this was the first time he ever took a life.
8 Figuring Out That Reiner Was Inside The Wall
When the Survey Corps went to Shiganshina so they could take back their land from the titans by sealing the hole the Colossal Titan made, they knew that the warriors were nearby but couldn't find them. Armin led the search and discovered that Reiner was inside of the wall. Levi quickly attacked him, and though Reiner didn't die, the Survey Corps could have been at a greater disadvantage if Reiner continued to hide until he was ready to fight.
7 Discovering That Reiner And Bertholdt Were Titan Shifters
Armin leading the search for Reiner in Shiganshina wasn't the only time he discovered him. Before that, he was able to find out that he and Bertholdt were the Armored and Colossal Titans respectively. After finding out that Reiner and Bertholdt were supposedly from the same region as Annie, Armin recalled that Reiner was with him when she attacked.
Despite her nearly killing Reiner, he was able to escape and she left him alone afterwards. It turned out that he had carved Eren's location into her hand. Upon realizing this, Armin and the rest of the Survey Corps strongly suspected him of being the Armored Titan, which he confirmed shortly after.
6 Saving His Friends From Seven Titans In Trost
When the titans invaded Trost, Armin and his friends were in danger. In order to save them from the seven titans that surrounded them, he came up with a plan in which most of them would lure the titans closer. Meanwhile, Mikasa, Reiner, Bertholdt, Annie, Jean, Sasha, and Connie killed them. Though Sasha and Connie failed to defeat the titans they were supposed to, Annie and Mikasa saved them, making it so Armin's plan was a success.
5 Becoming The Survey Corps' Commander
Towards the end of the manga, Hange sacrificed their life in order to make sure that the Survey Corps and Warrior Unit would be able to catch up to Eren. Before doing so, Hange told Armin that he would become the Survey Corps' commander. Though the Survey Corps only had a few members at that point, becoming their leader proved just how far Armin came from not even being one of the best soldiers in the 104th Cadet Corps. As their leader, he led his comrades into the battle against his best friend, helped end the Rumbling, and saved the world.
4 Seeing The Ocean
Ever since he was a child, it was Armin's dream to see the ocean. This was his reason for joining the Survey Corps in the first place.
He first learned about it from a book he read, and, after showing it to Eren, who didn't believe it could exist, he was determined to prove that it was real, especially since his parents were killed by the Military Police for wanting to see the outside world. Years later, he and Eren were finally able to see it, making everything that Armin worked for since childhood worth it.
3 Figuring Out That Annie Was The Female Titan
After his encounter with the Female Titan, Armin was able to discover that she was Annie. When thinking about how she knew what Eren looked like and that she spared his life, he remembered that she had Marco's gear after killing the two titans the Survey Corps experimented on. This led to them coming up with a plan in order to confirm that she was the Female Titan. If they hadn't, Armin also wouldn't have been able to discover Reiner and Bertholdt's identities.
2 Defeating Bertholdt By Sacrificing Himself
When the Survey Corps and Warrior Unit fought at Shiganshina, Armin and Eren teamed up to defeat Bertholdt. Armin came up with a plan in which he got Bertholdt's attention while Eren snuck behind him and attacked. While it was a success and Eren was able to use Bertholdt as a hostage, their plan also resulted in Armin nearly being burned to death. Luckily, Levi injected him with titan serum and fed Bertholdt to him, allowing Armin to inherit the Colossal Titan. They finally defeated one of their greatest enemies.
1 Stopping The Rumbling
When Armin and his comrades finally caught up to Eren, they fought against each other. He was taken to the coordinate, where he was able to talk to Zeke about the meaning of life. By understanding Armin's beliefs, Zeke and the other titan shifters that Eren controlled were able to gain their free will back and helped Armin's comrades end the Rumbling. Armin then fought against Eren. With the help of his friends, especially Mikasa, they were able to kill him, saving the world.