Eren was usually seen wearing a green shirt and a brown jacket. He had a fairly long, yet rounded face, and sizable, expressive green eyes. His eyebrows were sparse and often furrowed, which made him seem troubled or annoyed. His hair was short and dark brown, with his bangs that fell naturally into a middle-parted curtain-type style. His complexion also seemed somewhat tan.
Eren was always bent on joining the Survey Corps, despite having seen them come back all wounded. He always wanted to see what was beyond the Walls and had a pure hatred for those who disagreed with him.
When Doctor Yeager visits Mikasa's mother for checkups, he brings Eren with him. Eren encourages Mikasa to explore the woods outside her home. As Mikasa becomes frightened of these woods, he tells her she lived like the chickens her family raises. After she starts crying, he reassures her that the woods are not dangerous. Soon after, they encounter a pack of wolves, eating three men, and later report it to their parents. Eren grows closer to Mikasa as his father visits every ten days. Mikasa wishes she could see him more, and her mother then becomes sick, urging Doctor Yeager to visit every five days.
On one visit, Eren arrives with many injuries to his face, and his father explains that he must have fought with other children in his neighborhood. Shortly afterward, Eren explains to Mikasa that the people who fought him were content with living ignorant within the Walls and considered his views heretical. Mikasa wonders why Eren wants to see the outside world, and Eren tells her of the saltwater oceans, flaming waters, frozen plains, and snowfields of sand that exist beyond the Walls. However, he says that he and his other friend, Armin, will not be able to see the outside world for another five years when they can graduate from the Training Corps.
During Mikasa's visit to Shiganshina to deliver a hat, Eren is sent to meet up with her, and the two head to the south gate of Shiganshina to see the arrival of the Survey Corps. Eren ends up brutally attacked by two men after he hits one of them for insulting the Survey Corps. Eren does not visit Mikasa again, until her mother moves to Shiganshina to be closer to Doctor Yeager. Eren finds Mikasa sitting in a chair spaced out and takes her out for a walk. She asks Eren why he stopped visiting her. He shares with her that the Survey Corps has been shut down due to their lack of results and the gates have been sealed up. He tells her that he could not visit her anymore because he was busy building an airplane with Armin to fly over the Walls.
Later, Eren gives Mikasa a caged butterfly explaining to her that Armin had built the cage. He tells her that the airplane is finished and that he will leave during the night. Mikasa asks him not to die, and in response, he tells her that they will come back for her once they get settled outside the Walls. Before walking away, Eren wraps his scarf around Mikasa, telling her to take good care of it while he is gone. Eren is later killed when the airplane fails to fly up and crashes into a wall.
- Mikasa Ackerman - Eren had a strong relationship with Mikasa since Doctor Yeager had started visiting Mikasa's household to check on her mother's health, and he brought Eren with him every time he visited. As Eren and Mikasa had started becoming closer friends, Mikasa wished that Eren would visit her more often.
- Armin Arlert - Eren was Armin's childhood friend. Sometimes they played together from morning to night. They shared the same dream to travel the world outside of the Walls. Armin gifted Eren a butterfly container that he made.