Reiner Braun was born in Marley's Liberio internment zone to an Eldian mother, Karina Braun, and a Marleyan father. Relationships between Eldians and Marleyans were forbidden, preventing contact with his father. His mother urged him to become a Warrior candidate, believing inheriting one of the Nine Titans would grant them honorary Marleyan status and reunite their family. As a child, Reiner was physically weaker than Warrior candidates like Porco Galliard and Annie Leonhart, often mocked for his lack of natural talent. His primary strength was unwavering loyalty to Marley. Marcel Galliard, seeking to protect his brother Porco from the Warriors' shortened lifespan, manipulated the selection to ensure Reiner inherited the Armored Titan. This revelation during the Paradis Island infiltration shattered Reiner's self-confidence, compounded by Marcel's death saving him from a Titan. At age twelve in 845, Reiner participated in the operation to reclaim the Founding Titan on Paradis Island. Alongside Bertolt Hoover, Annie Leonhart, and Marcel, he breached Wall Maria, causing widespread destruction and loss of life, including the death of Eren Yeager's mother. Following Marcel's death, Reiner assumed leadership of the remaining Warriors, insisting they continue their mission. He, Bertolt, and Annie joined the 104th Training Corps, where Reiner adopted a "soldier" persona characterized by reliability, protectiveness, and camaraderie, earning him the role of "big brother" among the cadets. He graduated second in his class. This double life caused severe psychological fractures. Reiner developed dissociative identity disorder, oscillating between his Warrior identity and adopted soldier persona. Guilt from betraying comrades, particularly after killing Marco Bott, exacerbated his instability. During Marco's death, Reiner initially displayed distress but suppressed these emotions to maintain mission focus. His breakdown culminated during the Clash of the Titans arc, where he revealed his true identity to Eren Yeager. After returning to Marley, Reiner fought in the four-year Marley Mid-East War. By 854, his physical appearance reflected his trauma: he grew taller, developed a goatee, and lost significant weight, resulting in defined cheekbones. Chronic stress, guilt, and the physical toll of Titan transformations contributed to his prematurely aged and gaunt look. He suffered persistent nightmares, depression, and suicidal ideation, attempting to take his own life with a rifle. He halted this attempt only out of duty to protect younger Warrior candidates like Falco Grice and Gabi Braun, whom he viewed as a means of atonement. In the Final Season, Reiner served as Vice Captain of the Warrior Unit. During the war against the Mid-East Allied Forces, he prioritized protecting Falco and Gabi, even suggesting Falco inherit the Armored Titan to spare Gabi the Warriors' fate. He expressed conflicted views about Paradis inhabitants, describing them as ordinary people rather than "devils," unsettling his family and comrades. His encounter with Eren in Liberio's basement forced a confrontation about their shared past, culminating in Reiner witnessing Eren's declaration of war. Despite his trauma, Reiner retained strategic acumen and leadership skills. He analyzed situations methodically, formulated long-term plans, and maintained composure under pressure, qualities acknowledged even by adversaries like Eren. He often bore blame from fellow Warriors for mission failures, reflecting his perceived role as their leader and scapegoat. His Armored Titan, standing 15 meters tall, featured full-body armored plating except for joints behind the knees, elbows, and feet. Despite its weight, it demonstrated agility in combat, capable of climbing structures using hardened claws.