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The Armored Titan
The Deviant Titan that destroyed the inner gate of Shiganshina, allowing the other Titans to break inside Wall Maria's territory. Its entire body is covered in protective armor plating except for some weak points at its joints, which were used by Mikasa to damage it during its fight with Eren.
Reiner comes from a mysterious location simply referred to as his hometown (or homeland), and began a mission to attack humanity in the year 845. Little is known about his life prior to undertaking his mission, other than the fact that Bertolt and Marcel were his childhood friends. The exact nature of their group, and their mission is yet unknown. It is mentioned that their goal was to exterminate humanity, and that they are searching for both the Ape Titan and a "coordinate" which they believe may be Eren Yeager or possible to locate with the assistance of Historia Reiss.
At some point prior to the attack on Wall Maria, he was attacked by a Titan later revealed to be Ymir. In the chaos, his friend Marcel pushed him to safety and died protecting him. This event appears to have left him deeply traumatized, as he flashes back to it in moments of stress and expresses an almost suicidal desire to place himself into harms' way for the sake of others.
The Fall of ShiganshinaHe first appeared in the year 845 as the Armored Titan, participating in the attack on Shiganshina. During the chaos of the initial breach, he entered the city in his Titan form and made his way across it to the interior gate. Once there, he charged towards the gate and smashed through it, allowing Titans to invade Wall Maria. At some point during the abandonment of Wall Maria, he passed himself off as a refugee and infiltrated human territory. He was seen with Bertolt as the campaign to retake Wall Maria was announced, wearing a shocked expression.
Joining The Military
Two years later after the destruction of Wall Maria he enlisted in the military and becomes a member of the 104th Trainees Squad. When Eren struggled with the Three Dimensional Maneuver Gear and began to look for advice from the other trainees, Marco Bott advised him to approach Reiner and Bertolt for help. Though initially unfriendly towards Eren and Armin, he quickly warmed up to them after learning of their being from Shiganshina District and still willing to enlist in the Survey Corps. He shared with them his desire to return to his lost homeland, and would become a surrogate big brother to both.
During hand-to-hand combat training, he advised Eren to take his duty as a soldier seriously. Noticing Annie attempting to sneak away from training, he intercepted her and goaded her into sparring with Eren. After seeing her easily dispatch Eren, he hesitated to take her on himself....but was goaded into it by Eren and quickly defeated. This introduction would lead to Eren and Annie becoming close, with her tutoring him in unarmed combat.
Reiner was noted by Keith Shadis to have exceptional mental and physical fortitude, as well as great ability to gain the confidence of his comrades. He would eventually graduate 2nd in the class, finishing just behind Mikasa
The Battle of Trost
As part of the defensive effort of the military after the second attack of the Colossal Titan, he eventually found himself with the front guard. However, with the supply team stuck in the headquarters, he was essentially immobilized with many of his teammates on a rooftop. Joining Jean's group in the rush to the HQ, he was among the survivors. With the subsequent arrival of Armin and Mikasa, by way of the Rogue Titan, he helped to clear and secure the supply room.
Initially skeptical, Reiner suggested they help the Rogue Titan when it was being overwhelmed by a group. Jean was hesitant and disagreed with his suggestion, stating they barely made it with their own lives and could use the opportunity to escape. However, it turned out not to be necessary when it broke free at the sight of the Titan that killed Thomas Wagner. He was among the first people to know about Eren's Titan form, when he emerged from the giant's body. Later this same day, after seeing what appeared to be Titan vapor from inside the city, he and a small group scaled the wall to see what was happening. Over the wall they found Eren protecting Mikasa and Armin from cannon fire, with a shield made from the torso of his Titan form.
- In a post on his blog, author Hajime Isayama stated that the Armored Titan's physique was based on that of professional wrestler and mixed martial artist Brock Lesnar.