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Before the fall of Wall Maria, the Arlert family lived a peaceful life in Shiganshina District. One man in the family kept a secret book which contained forbidden knowledge of the world beyond the Walls, with descriptions of fiery lakes and snowfields of sand. Eventually, the man gave this book to his grandson Armin Arlert. When he was nine years old, Armin showed this book to his friend Eren Yeager, inspiring him to see the outside world.
When Shiganshina District is attacked by the Colossus Titan in the year 845, at least four members of the Arlert family escaped: Armin Arlert, his parents, and his grandfather. When Eren and Mikasa run to their house in the carnage to see if their mother is safe, Armin warns Garrison soldier Hannes of their peril, saving them from being eaten by Titans.
One year after Shiganshina District is overrun by the Titans, the Military issues a culling disguised as a reclamation expedition. In this expedition, Armin's grandfather and parents are killed. Armin would spend the next year with his friends Eren and Mikasa, working the fields to provide food for the people of the Walls.
In the year 847, Armin Arlert joins the 104th Training Corps. He shows little physical strength but exceeds with intellectual skills. On the day before graduation, Armin is nearly eaten when Trost District is attacked by the Colossus Titan and many Titans enter the city. He is saved by Eren, who is thought to have died in the process. However, it is later discovered that Eren survived due to his Titan powers. With this knowledge, Armin formulates a plan to save Trost with Eren's powers, and with the help of the trainees and the Garrison the plan succeeds.
After graduation, Armin joins the Survey Corps and partakes in the 57th expedition beyond the Walls, where he discovers that the Female Titan that invades the expedition is in fact his former trainee comrade Annie Leonhart. After bringing this information to Commander Erwin Smith, Annie is successfully captured in Stohess District.