|Alongside all those different people stood us. The days I spent there were true hell.|
|— Reiner Braun recounts the years he endured on Paradis Island|
The Boy Inside the Walls (壁の中の少年 Kabe no Naka no Shōnen?) is the 4th and final chapter of the 23rd volume and the 94th chapter overall of the Attack on Titan manga, written and illustrated by Hajime Isayama.
The Warriors return at long last to their hometown, reuniting with their families. At the Braun family residence, Gabi's heroism is praised and Reiner tells of the horrors he experienced living among the devils of the island.
Later that day, Reiner recalls his youth when he trained as a Warrior so that his Eldian mother and Marleyan father could live together. Faced with many struggles and shortcomings, Reiner's dream kept his resolve alive.
Arriving at the outskirts of Liberio, an ecstatic Gabi celebrates returning to their hometown while an uneasy Falco observes Deputy Chief Reiner. Falco recalls that Reiner's failures on Paradis Island were overlooked because of his unquestionable loyalty to Marley, and he is perplexed by Reiner's plea to "save" Gabi from inheriting the Armored Titan. He begins to question whether he can be trusted. Gabi taunts Falco, noticing his suspicious behavior, and the others are amused at their banter.
The Warriors march through the streets of the main city, watched by the disgusted townsfolk until they arrive at the internment zone where they are greeted by their families. Zeke reunites with his grandparents, who are proud of his accomplishments, though Zeke says he is not finished with his duties yet. Reiner and Gabi are greeted by their relatives, who praise Gabi for her heroic accomplishments at Fort Slava, and Falco is distracted by the sight of a large number of wounded Eldians who are being escorted away. Koslow explains to Falco that they were left traumatized by the war and are being taken to a nearby hospital for treatment, as they have no families to return to. As the officer happily begins tormenting the soldiers, Falco asks his family to go one ahead of him, and he attempts to help one of the injured soldiers, noting that the soldier's armband is backwards.
Later, Reiner and his family have dinner at their house. Gabi recounts how she single-handedly destroyed the armored train at Fort Slava, and Reiner's mother, impressed, asks Reiner if she will become a Warrior. Reiner assures her that, based on Gabi's record, she will likely become the next Warrior. Karina is delighted to hear that a second member of their family will become a Warrior, noting that all that is keeping them from being happy is the inhabitants of Paradis Island. This prompts a number of questions for Reiner from his family about the people of the island. Although Gabi insists that Reiner should not be forced to answer any such questions, Reiner agrees to tell them about his time in the military on Paradis Island.
Reiner informs his family about the hardships he endured while among the evil Eldians on Paradis Island. He begins by recounting his experience with one Eldian who had stolen a potato and attempted to bribe her instructor when she was caught. He describes the people he met during his time on the island: an idiot, a hypocrite, and an over-responsible jerk. Reiner makes special mention of an Eldian who had been obsessed with revenge, and the people who followed him no matter what he did, and concludes by saying that it had been a nightmare for him, Bertolt, and Annie to be stuck with such a differentiated group of people.
Gabi is puzzled by Reiner's choice to describe the people of Paradis Island as "different" but his mother is quick to remind her that all of the inhabitants of Paradis Island are evil. She tells Gabi that the people of Paradis Island could choose to leave the island at any time and destroy the world with their Titans, and that they, the good Eldians of Marley, are the only ones with the power to stop such a thing from happening.
Later, Reiner lies in his old room and recalls his childhood. Reiner never knew his father, a Marleyan man, as intermingling between Eldians and Marleyans was strictly forbidden. As a result, he chose to join the Warrior program in the hopes of becoming one of the Nine Titans so that he and his mother would become honorary Marleyans and would finally be able to reunite with his father. Despite performing unexceptionally in the program, Reiner was among the seven children chosen to be a Warrior candidate.
One day after a training run, Zeke, another candidate, announced to the candidates that an attack on Paradis Island was being planned to commence in a few years, and that the time for the next Warriors to be chosen was drawing near. However, only six of the seven candidates would be chosen to inherit their Titans all at once. Reiner was ecstatic to hear that he would soon become a Marleyan. Galliard, annoyed at Reiner's optimism, reminded him that the only positive trait he had demonstrated as a candidate was his intense loyalty, which would not be enough to gain one of the seven Titans in Marley's possession.
Reiner, believing Galliard to be making light of the Warriors' mission, threatens to report him for treason, resulting in Galliard angrily striking him. As Marcel separates the two, Galliard proceeds to mock Reiner before leaving with Zeke. Bertolt helps Reiner up, who despairs at the thought of not being chosen for one of the Titans. Bertolt reassures him that there is still a chance for him to be chosen based on his loyalty. As the two go to catch up with their fellow candidates, Bertolt asks Reiner if he is okay with becoming a Marleyan if it means he only gets to live thirteen more years. Reiner responds that, in thirteen years, he will be a hero for having defeated the devils living on Paradis Island and will make his parents the proudest of all.