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Marco has short black hair which he keeps parted down the middle. He wears a black Japanese school uniform.
Although Marco is not shown to be very talkative, his interactions with his classmates have proven him to be very kind, and he will often go out of his way to help others. He is also shown to be very popular with his classmates, inadvertently convincing them to elect him class president while giving a speech on why Jean deserved the position.
While waiting for class to begin, Marco sits at his desk reading a book. After roll call, the teacher tells each student to stand up and share their dream with their classmates. When a boy named Eren Yeager tells the class that it is his dream to drive the Titans out of the world, he is mocked by another boy named Jean. When Eren gets mocked, a feud begins. A girl named Mikasa Ackerman intervenes and stops the fighting. After Mikasa returns to the classroom from consoling Eren outside, she is worried about how he will get along with his classmates and decides to ask her classmates' impressions on him. Marco admits that he is afraid of the Titans and he does not think that he and Eren would get along.
Later that day, a Titan tries to eat Eren. Like the other students, Marco does nothing to help Eren because he is too scared. Eventually, Mikasa stands up to the Titan and saves Eren. When Eren regains consciousness, Mikasa tells everyone that Eren puts himself in harm's way to keep everyone safe. At first, everyone stands there with blank expressions of their faces, but Marco admits that Eren was incredible and that he would not have to fear the Titans with Eren around.
When prompted to sign up for an extra curricular club at school, Marco decides to join the Student Government and is frequently seen working to help the student body. When Jean starts bemoaning how he is jealous of how most other students are pairing up, Marco tries calming him down and telling him to not worry about it. Later, he suggests that Jean find something he is passionate about and unknowingly spurs him to run for student body president. However, Marco is caught by surprise after the student body rejects the eventual winner Ilse Langnar and selects him as president instead; when asked by Jean what he did to stand out as a candidate, Marco admits he did nothing out of the ordinary and sincerely thought Jean was the most qualified to win the position.
One day, Marco refuses to walk with Jean to his family's bookstore and leaves on his own; seemingly due to him not returning several of Marco's things he had been lent, Jean tries to return them but to no avail. Eventually, Marco is going through one of the lost and found bins at school when Jean arrives begging for forgiveness. Marco is perplexed by this behavior, but admits that he has an eraser Jean had lent him but never got to return it either. The two make up and Jean offers to help go through the bins; unfortunately, he knocks the contents of one over and begs forgiveness again, much to Marco's irritation and embarrassment.
- Jean Kirstein - Becoming friends on their first day of junior high, Marco is usually never seen too far from Jean. He often acts as the voice of reason when Jean starts bemoaning his social placement within the school.