Reiner Braun was born to an Eldian mother, Karina Braun, and a Marleyan father within Marley's Liberio internment zone. Strict laws forbidding Eldian-Marleyan relationships denied him any contact with his father. His mother urged him to become a Warrior candidate, convinced that inheriting one of the Nine Titans would earn them honorary Marleyan status and reunite their family. As a child, Reiner lagged physically behind fellow candidates like Porco Galliard, Annie Leonhart, and Bertolt Hoover, enduring mockery for his perceived weakness yet showing fierce loyalty to Marley. His selection to inherit the Armored Titan at age ten stemmed from Marcel Galliard's manipulation; Marcel sought to shield his brother Porco from the Titan's thirteen-year curse. Learning this truth during the Paradis Island infiltration mission shattered Reiner's confidence. At twelve in 845, Reiner joined the mission to reclaim the Founding Titan on Paradis alongside Bertolt Hoover, Annie Leonhart, and Marcel Galliard. Shortly after arrival, Marcel sacrificed himself to save Reiner from a Titan, fueling Reiner's guilt and driving him to take command. On Paradis, Reiner confronted his estranged father, who rejected him and Karina as "Eldian devils," destroying Reiner's hope for reconciliation. Reiner, Bertolt, and Annie breached Wall Maria, causing catastrophic loss of life. They infiltrated the 104th Training Corps, where Reiner graduated second. He cultivated a dependable, protective "big brother" persona among the cadets while covertly advancing Marley's goals. This duality fractured his psyche, especially after murdering comrade Marco Bodt, who overheard their secret identities. Reiner forcibly removed Marco's vertical maneuvering equipment, resulting in Marco's death by Titan. This act intensified Reiner's dissociative identity disorder, causing him to alternate between his ruthless "Warrior" self and a "soldier" persona that genuinely believed in protecting Paradis. His Armored Titan stood 15 meters tall, encased in hardened skin plates nearly impervious to conventional weapons. Weak points included exposed muscle tissue behind the knees, elbows, and feet. Despite its armor, the Titan displayed significant agility during battles with Eren Yeager. Following the mission's failure and four years fighting in Marley's war against the Mid-East Allied Forces, Reiner's mental state deteriorated. He suffered chronic nightmares, memory flashbacks, and suicidal ideation, restrained only by duty to his family. By 854, his physical decline was evident: he had lost weight, grown taller, and sported a goatee. While an antagonist, he acknowledged the humanity of his former Paradis comrades, expressing remorse for his actions and confessing directly to Eren Yeager his responsibility for Eren's mother's death. Reiner possessed strategic acumen and leadership skills, devising complex mission plans and maintaining composure under pressure. His ability to analyze long-term consequences was noted by adversaries like Eren. Even as a prisoner of the Survey Corps, he prioritized mission secrecy over self-preservation.