Armin Arlert

Armin Arlelt (アルミン・アルレルト) is a childhood friend of Eren's and one of the main protagonists in the series.Although physically more frail that the rest of the soldiers he has proven his skills through his innate capability for forming strategies and his high intelligence.

Plot
Armin was conisdered physically the weakest and graduated below the top ten. Although his plans has saved Eren and Mikasa several times, he doesn't realise it and despises himself for being reliant on them to help him out. This occured when they were children when he was bullied by everyone else for wanting to go outside the walls and was considered a heretic. Armin supposively blamed himself for allowing Eren to sacrifice himself to the bearded titan. In one of the chapters as he was going through the plan, Mikasa tells Armin that his strategies has always saved their lives and he should have more faith in himself.

Sometime later before joining the Scouting Legion, Armin suspiciously took notice of Annie in possession of Marco's Three Dimentional Maneuver Gear as her own, which this he would use later on. After joining the Scouting Legion with Eren, Mikasa and the other graduates(excluding Annie), they were confronted by the sudden appearance of the Female Titan, seemingly very intelligent with military formation. She attacked Armin with his hood and when it was removed, she saw he wasn't Eren and left him alone. It was then he realised that Annie was indeed the Female Titan.

Arc 2: Armin's Theories and The Female Titan Exposed
In Chapter 31, Armin asks Annie for her help in hiding Eren because she was in the military police and they needed her to let them pass through Wall Shina without suspicion. It worked and Armin explained that he wanted to take advantage of the geography there rather than use the 3DMG to bust through. As she was leading them to a hiding place, Annie refuses to go inside to help them escape and soon asked him why he was looking at her the eyes of suspicion. Armin then took the time to ask her why she was in possession of Marco's 3DMG. He recognised it not too long ago because they would often do routine check-ups to make sure they are in pristine shape. Annie confessed that she found it after Marco died and that she did use her own 3DMG to kill the two captured titans. She then asked him that if he came to all those conclusions a long time ago, why he didn't take action then. Armin reasoned that he didn't take action because he thought there was still something wrong with his train of thought. It was when Annie refused to kill him that things turned out the way it did.

In Chapter 32, Armin attempts to capture Annie by firing the signal from his flare gun letting the townspeople capture her. As the people tried to restrain her, she managed to injure herself with a blade hidden in her father's ring. Mikasa took the time to grab borh Eren and Armin and take them underground as Annie in her tian for attempts to capture them. When Armin learns his plan had failed he attempted to start plan B, tried to encourage Eren to fight Annie. Eren tried to transform into his titan form, but he was unable to do it. It was only then Mikasa and Armin learned why, he was reluctant to fight Annie again due to a previous fight he had with her that he lost. Realising no other option but to remind Eren the world is a cruel place, they prepared to take Annie on by themselves.

In Ch.34, Armin tells Jean that Eren's summons to the king is cancelled and that Annie is the Female Titan. She's being detained underground after encasing herself in a crystal to avoid interrogation. Armin couldn't understand how the walls were built in the first place because there was no stones or bricks around. He comes to theorize that the titan's hardening abilities the reasons for the walls being built. He believes it could explain how Annie was able to encase herself in a crystal.

In Ch.37, Armin and the others learn from Lt. Hanji that the only way they could repair the walls is if Eren can use his hardening abilities to help fix Wall Rose. Pastor Nick tells them about a girl in the Scouting Legion knowing more about what's going on and it's up to her to decide if she'll talk. Armin then recognized Christa beacause she was often seen with Ymir.