Mylius Zeramuski was of average height with light hair. He wore the typical uniform of a member of the 104th Training Corps.
Mylius seemed shy and cautious; he warned Eren when he was running off to avenge Thomas Wagner for fear of his safety and inexperience. However, he showed bravery when charging in with his squad to save his comrade.
During the Battle of Trost District, Mylius is assigned to a middle guard squad in Trost District. Shortly after the squad's deployment, the group is attacked by an abnormal Titan, which results in the death of Thomas Wagner. This causes Eren Yeager to attack it in a rage, despite Mylius' warnings. Eren is injured, and the rest of the squad is killed in short order trying to help him.
Armin later reports Mylius' demise to the remaining members of the 104th Training Corps, saying he died valiantly in the line of duty.