Reiner Braun appears in the first season of Attack on Titan, during the training of recruiting new soldiers for the three sects, Survey Corps, Military Police, and Garrisons. He is a graduate of the 104th Trainees squad and joined the recon corps along with his fried Bertolt Hoover and Annie Leonhart.
Until the big reveal in the third season, that he is a betrayer among the others, he is actually a Marleyan, and the armoured titan he was considered to be one of the closest friends of Eren and others.
Back in Marley, light is thrown on his former life, the reason he became the armoured titan, and his past. He is an illegitimate son of a Marleyan and an Eldian but he grew up in the internment zone and was later on chosen by the Marleyan Military to be a warrior for being an honourary Marleyan who could live among the mainstream if he chose to kill the Eldians and the entire race.
When he was ten, he was vested with the power of the armoured titan and was said to infiltrate the Paradis Island and become one of the soldiers along with the Eldians residing there, to gather information and then plot to erase them.
Their main motive was to capture the Founding Titan that they knew was among them. His lifelong sacrifice and pursuit started with his belief that he could meet his father, who was a Marleyan, and if he became a soldier in pursuit of erasing Eldians, his father would accept him.
His childhood has been shown in a series of flashbacks where his friendships with Annie and Bertolt are shown, he is the one being left out because of a feud between him and Porco, who was the brother of Marcel, the latter of whom went on to become the Jaw Titan.
Child-rearing: Marleyan Perspective, racism, and propagandist agendas
The children who had Eldian blood in them were raised in the internment zone, brainwashed into training themselves to become warriors to fight against the Eldians, only then will their existence be validated.
This ruthless manipulation of their minds embedded racism in their thinking process is the reason for their stunted reasoning and maturity, which is overwhelmingly shown in the characteristics of Reiner, Bertolt, Annie, and mostly Gabi.
Reiner, as he grows old, regrets his life choices from the start, the endless pursuit to be accepted into a society to be used as a pawn to fight with his own kind is what internalised systematic oppression and racism results in, whose victims are the children of Marley.
Plunging a child into the face of death and political conflict with inherent racism is just the worst possible way to bring up a child. Reiner’s mothers always referred to the Eldians on the Paradis Island as “Devils” who deserve to be exterminated.
As he grew and saw his friends die in front of him, he still didn’t give up even though he should have, which resulted in him doubting himself and having self-hatred into him. His loneliness paired with the facade of keeping up appearances to others as their elder brotherly figure, the emotions and the turmoil with his own betrayal towards them conflicted with his personality and plunged him into depression.
Emotional burdens and conflict: “Whose side am I on?”
Reiner's double act of being a dear friend and a big brother to the trainees of the 104th squad, while plotting to kill everyone, including the ones he called friends started to affect him.
When he let Marco die because he eavesdropped on his and Bertolt’s conversation was the last straw that caused him to snap, tired of having different personalities, he started to lose sight of his purpose of existence and identity. Sometimes he believed he was a loyal Eldian Soldier but also he kept reminding himself of his loyalty to Marley.
The loss of identity and distant family and a conflict in his ideals et his entitlement to his loyalty to Marley made him an extremely depressed and suicidal person who regretted every action that followed since he appeared in the Shiganshina district in his Armoured Titan form.
These deep-seated psychological problems are rooted in his dependent and manipulated upbringing, the problematic racial ideals that made him think that being a warrior would give him the validation that he needs to survive.
Despite all the shortcomings and the tragic losses he deals with, Reiner Braun is a very intelligent and empathetic who despite giving up on his life, cares for the people related to him and fights Eren for their sake, despite not hating his previous comrades as well.