One by One

Koko (個々) is the 2nd chapter of the 4th volume and the 15th chapter overall of the Attack on Titan manga, written and illustrated by Hajime Isayama.

Overview
Three years before humanity's victory at Trost, Armin along with Eren and Mikasa are in the fields overhearing a triggering conversation between Military Police officers. Not long from then, Armin is in boot camp getting through initiation by being intimidated by their instructor. Their instructor does the same to a few other cadets. At around sunset, the group of cadets speculate Sasha running laps as her punishment for eating a potato in front of her peers. Eren and Armin are quizzically asked by other cadets about their experience with the Colossus Titan. Jean decides to pitch in, starting a fight with Eren. As soon as the dinner bell sounds, the pair decide to call it quits. Mikasa walks past Jean, catching his eye, but after spotting her with Eren, Jean feels somewhat betrayed. Sasha finishes her running and Krista offers her some nourishment before meeting a fellow trainee.

The next day, the cadets work on their Vertical Maneuvering Equipment skills. Unlike most, Eren finds that he is incapable of balancing.

Summary
Two years after the first attack of the Colossus Titan and three years before their graduation from the 104th Training Corps, Mikasa, Eren and Armin are working in a field. A small group of Military Police officers are walking by and discussing productivity. The officer in charge is complaining because the fields are behind schedule. His subordinate says that most of the workers were kicked out and sent to recover Wall Maria, likely killed. Armin, glaring at the soldiers, tells Eren and Mikasa that his parents were sent on that mission not for recovery, but so the government would have less mouths to feed.

A short time later, Armin is now in boot camp. The drill instructor, pacing through the ranks, singles out Armin, asking his name. After a short, intimidating speech, the instructor moves on to another trainee. Two nearby officers recall their time in boot camp, with one questioning the point of intimidation. The other answers that it is used to cultivate the necessary qualities of a soldier. They notice that the drill instructor seems to skip Annie and some other trainees, leaving one to say that these soldiers have been through her rite of passage. The instructor notices the look on a lot of the faces, and assumes it was because of the attack from the Colossus Titan. Now onto Jean, he asks why he is here. Jean answers to join the Military Police and live in the Interior. With that, the instructor head butts Jean, causing him to fall. The instructor then adds insult to injury and says that he never gave him permission to sit, concluding by stating that someone who folds in training won't be allowed into the MPB. He asks Marco the same, Marco answers that he's from the town of Jinae and wants to join the Military Police so that he can be of service to the King, to which the instructor states that the King's got no interest in his scrawny body. The next up is Connie, who mistakenly uses the wrong hand in his salute. When giving him a hard time, the instructor notices Sasha, who is eating while in formation. He rushes over to her and asks what the big idea is. She's still chewing her food which causes her not to answer, so he gets louder. He asks what she is eating. She replies that it is a steamed potato, which she decided to eat before it got cold. The instructor glosses over the fact that is was stolen, instead asking why she is eating a potato. As the situation gets more tense, Sasha breaks the potato in two unequal portions, offering "half" of it to the instructor.

Now out of formation and at the barracks, Eren and others watch as Sasha runs laps well into her fifth hour and having to skip lunch, her reaction about having to skip lunch being much stronger than having to run "'til she was almost dead", which she took quite literally. They begin to talk more about where they are from. When the conversation gets to Eren, he tell them he and Armin are from Shiganshina District, which surprises the others and causes him to constantly be asked about the Colossus Titan. Later during mealtime, Eren briefly describes the Colossus Titan and Armored Titan to those gathered around him. As more and more questions come, Eren starts to think of his mother being killed and he gets nauseous. Marco and Connie notice and apologize for making him talk about it. Regaining his composure, Eren dismisses it and says that he can't wait to fight and kill the Titans. At hearing this, Jean accuses Eren of faking it while actually shaking in his boots. Deciding to let it go, the two apologize for the short argument. Mikasa walks by, which grabs Jean's attention. He tells her that he's never seen a face like hers before and that she has beautiful black hair. She thanks him and walks away. Outside, she catches up with Eren, with Jean following to watch Mikasa's departure. Eren denies almost getting into a fight and tells Mikasa that her hair is getting too long; it could cause her to get into an accident. She asks him how long she should cut it. Seeing this, Jean wipes his hands on Connie's shirt. Connie angrily asks why he is using his shirt as a napkin and asks what he wiped on it. Jean replies that it was his faith in humanity.

Sasha, now done with her laps, is on her way to her bunk when she collapses. Krista walks nearby with a little bit of bread and water, causing Sasha to lunge at her, grabbing the bread. Krista says that she brought it for her and asks if she wants any of her water. Grateful, Sasha sees Krista as a goddess. Another trainee stops by, asking why Krista is bothering to help Sasha and what the point of her "good deed" is. Krista replies that she wants to be thought of as helpful. The trainee helps getting Sasha to her bunk, but makes it clear that she's only doing her "good deed" to make use of Sasha's stupidity.

The next morning, the students start their Vertical Maneuvering Equipment training. Suspended by wires meant to resemble maneuvering equipment grapples, they will test their balance. Mikasa, Jean and Connie are doing well, but Eren seems to be having problems, leaving him embarrassed.