Armin has big blue eyes and medium length blond hair with bangs cut straight across. He usually wears the typical school uniform excluding the black jacket, although he does wear it sometimes. Armin usually wraps himself in his favorite futon, Bigbee, wherever he goes. The futon is lined with furry white and its main design consists of a pink with yellow polka dot kitten heads. On some days, Armin will only wear the futon wrapped around the top of his head instead of his entire body.
After letting down his team in a dodge ball tournament, Armin remained home for a long time. He had spent this time studying RPG games, and learning about worlds of fantasy. Armin progressively enjoys spending his time outside after being bribed by Eren and Mikasa. He displays shy behavior from time to time, but has a very mature point of view. His most notable trait is his extreme hatred of the cold. This causes him to rarely go to school during the colder weather. He spends a majority of his time at home, in a room so overheated, it caused Eren and Mikasa to become sweaty almost instantly, with Mikasa even becoming woozy.
Armin finds his friends Eren and Mikasa looking for him. Armin is reluctant to leave the safety of his room, until Mikasa mentions that they have brought food. Armin welcomes his friends to his home with open arms and sits in his overheated room. Eren notices a tear in Armin's futon, but Armin insists on keeping it untouched. Suddenly, Armin's balcony doors swing open and Levi emerges, snatching Armin's ripped futon away from him. Armin follows Eren and Mikasa out the house, the two trying to find where Levi went. Armin runs after them to school and they find the Attack Junior High Scout Regiment, the apparent club in which Levi is in. Levi eventually arrives and finishes stitching up Armin's futon, then returns it. Armin appreciates the kind gesture and takes part in the Attack Junior High Scout Regiment club from then on.
Armin participates in the game of dodgeball with Jean and the team. Having noticed that Eren is nowhere to be found, Armin reasons that Eren overslept again. Hearing about Eren's absence, Keith Sadies threatens the group in making them as Titan food if he founds anyone slacking off. Later in the game, he notices Sasha slurping ramen in the middle of the game. Armin and the others manage to win the first match but upon realizing Mikasa's depressing state he comes up with an idea. In order to cover up Eren's absence, he puts a wig on Jean's head for him to pretend as Eren. Armin comments out the similarities between the two and declares that in this way it can be a possibility to cheer up Mikasa again. With Mikasa's denial, Eren bursts out into scene as everyone proceeds onto the last match of the game.
Along with the rest of the group, Armin heads to the Wall Cleanup Club because of the unauthorization of the Scout Regiment. On their way to the club, Eren notices his weird head accessory, asking if the heat does not bother him. Armin reveals his new futon design which is actually a mesh structure that holds up water in order to keep him cool. Later that day, Armin ends up cleaning the windows with the gang under Rico's orders. Acknowledging the Wall Cleanup Club's secret weapon, Armin proceeds to get the job done and later is introduced to omni-directional mobility gear. Witnessing Eren's continuous failure, Armin points out a possible cause with the gear's broken mechanics. Afterward, Hannes falls, so Rico declares the broken gear and everyone is dismissed.
After Eren's success and definitive happiness on using the gear, Armin and the rest witness a horrible scene of the Titans vandalizing the Walls.
The day of the test results, Armin surprises his classmates by scoring 100. After class, Sasha seeks his help for remedial class. Armin accepts and proposes to Conny to join them.
When Jean brags to his classmates about receiving a love letter, Armin points that it may be a prank since the letter is not signed. Armin and the others later accompany Jean to a meetup at the park. Annie screams and summons her Titan friend, and Armin questions if she can understand Titan language.
Armin and his classmates compete against the upperclassmen in the school's Sports Day. Participating in the last game, shoulder wars, Armin's team defends Eren's from Mitabi's team, lunging themselves at them. Armin holds off Mitabi, encouraging Eren to fulfill his responsibility and to press forward.
The upperclassmen plan a sudden "Test of Courage" event, and Armin goes with Eren's group to the art room, cowering behind Mikasa. Hearing a disembodied laugh coming from a painting, Armin fears it to be the "Laughing Mona Lisa," No. 1 of the school's Seven Wonders. They all flee from the room in horror after they see the face in the painting morphing and giving a sinister grin.
Arriving at the music room, the group suddenly hears the piano playing by a figure and Armin points it to be No. 2 of the Seven Wonders, Beethoven playing Symphony No. 5. Everyone starts throwing music instruments at the figure, and Eren is separated from his group after running off on his own. Catching up to him on a stairwell, Eren starts screaming to them about a mirror, and Armin points it to be the cursed infinity mirror, No. 3 of the Seven Wonders.
Finally arriving at the gym, they are met by No. 6 of the Seven Wonders, the "Headless Dribbling Ghost." Armin and the others run around as most of the Seven Wonders they saw earlier appear and start chasing after them. As a giant cannon chases everyone, Levi is irritated by all the commotion and hits the cannon, then reattaches his head to his body, revealing the test to be a prank orchestrated by the upperclassmen.
Armin visits the summer festival with his classmates. He later participates in die-cutting with his friends, then watches in enjoyment as more festival fireworks are set into the sky.
After the student council scandal, an election is held for a new president. Armin helps Eren with making the design of his election poster, with the latter praising his skill. Eren then guides Armin for a few more tweaks, which result in a disfigured-like design of Eren's face. Armin and Mikasa are shocked, but Eren likes it and believes it will get him to win the election. He excitedly asks Mikasa of her opinion and Armin thinks to himself that Mikasa should tell him what she really thinks, but she replies that it looks good, to Armin's horror.
When it is Eren's turn for his final presidential speech, Armin tells him not to say anything weird. On stage, Eren states that his reason for becoming president is to destroy every Titan in the school. The crowd starts booing. Seeing this, Armin talks to the crowd and advocates for Eren's presidency, telling them if they combine Eren's conviction with the student council's remaining authority, restoring mankind's right will no longer be a dream. Everyone starts cheering, before Eren started calling them cowards and to just vote for him, ruining his chance and making Levi kick him in the face, to Armin's shock.
With the school's 104th festival approaching, Eren decides for his class to participate in the gourmet competition by doing cheeseburgers. At the chee-burgs stand, Armin is chosen to be the salesperson. He welcomes the customers to try their tasty cheeseburgers. Attracted by his costume and his way of promoting, they are all drawn to the chee-burgs stand.
Seeing their rivals' pot of rice is nearly ready, Armin suggests a limited-time sale to Eren; doubling the amount of cheese. When their rivals are about to open their rice pot's lid, Armin starts promoting for the double cheeseburgers with double cheese for the same price, attracting more customers. However, everyone evacuates the area when Titans suddenly invade it and steal all of the food stands.
Armin accompanies Eren when the latter heads to the Titans' building to retrieve their chee-burgs. They are chased down by Titans inside their giant hallway before Hannes ushers them to a hole in the wall. They find Rico and Ian have brought the Wall Cleanup Club's gear with them. They sneak out after Hannes distracts the Titans. In the principal's office, they find a huge key lodged in the middle of a walled town, and a door with a matching keyhole is further ahead. Armin helps Eren in pulling the key, and reminds him of his goal.
After Eren successfully carries the key and opens the door, they enter to what seems to be the gym, with a group of Titans inside it standing around all of the stolen food stands. As Titans surround them, they are saved when a group of upperclassmen distract them by playing on a stage. Armin then realizes that they are in fact inside the principal's lunch box when the Colossal Titan opens the roof. He screams in horror after Eren is eaten by principal, but it spits him out, and Eren reveals he had retrieved a chee-burg. Armin later celebrates their victory with his classmates.
- Eren Jaeger - Armin and Eren were friends since childhood. Eren is caring, brother-like, and supportive towards Armin and his decisions.
- Levi - Armin was initially unsure of Levi's forceful personality, but later realizes that he does things out of kindness. This is shown when Levi takes Armin's futon away from him, but returns it back after fixing a tear in it.
- Sasha Braus - The two are classmates and get along well. Armin is very kind and helpful towards Sasha, shown when he gladly accepted to help her studying for her makeup test. However, Armin can be critical towards her, being disappointed that she preferred to eat ramen than playing seriously during the dodgeball match.
- The name "Armin" may have multiple meanings, either from an old German word meaning "whole," or "Herman," meaning "soldier."
- Hajime Isayama chose the name "Armin Arlelt" since it sounds similar to the word "aluminum" and because starting both names with the same letter makes it easier to remember.
- The voice of Armin and the narrator of the anime are shared by the same voice actors in Japanese and English, Marina Inoue (in Japanese), Josh Grelle (in English).