Mike Zacharius is a third-year student at Attack Junior High School and a member of the Scout Regiment club. He possesses blond hair with bangs covering his eyes, a thin mustache, stubble, and faint sideburns, lending him an older appearance than his peers. He typically dons a green hoodie over his school uniform. His defining trait is an extraordinarily sharp sense of smell, which he employs to assess people and situations, reacting with smirks or tears without verbal elaboration. This habit extends to evaluating potential club recruits by smelling them. Despite frequent involvement in school events, he remains entirely silent throughout the series.

He participates actively across the school: observing first-year entrance ceremonies, attending club sessions for recruitment, awaiting exam results anxiously, and teaming with Levi during Sports Day competitions. He assists in organizing the Scout club’s "Test of Courage" night event, attends student council election speeches, and joins biology club sessions led by Hange. During a school festival disrupted by Titans stealing food stands, he disguises himself with Levi and Hange in a band, performing onstage to distract Titans while first-years escape the principal’s office.

In the manga adaptation, his germophobia surfaces—he refuses food not prepared by family or relatives, joining the cooking club only to observe. He recognizes Nanaba’s culinary talent but avoids tasting her offerings. When she presents him a rice ball, he departs abruptly without explanation. Later, during a bug-collecting trip with Hange, he detects food prepared by Nanaba and Gelgar in the woods. Confronted about his refusal to eat, he reveals his germophobia. He eventually overcomes this fear by tasting Nanaba’s rice ball, praises its flavor, and rejoins the cooking club to learn from her.

Within club dynamics, he serves as a Survey Club co-leader alongside Hange and Levi. During an upperclassmen vs. first-years competition, he acts as general for the upperclassmen, breaks free from a Titan net trap, admires but counters Reiner’s tactical sacrifice, and contributes decisively to the upperclassmen’s victory.

His relationships include mutual respect with Hange, who tolerates his peculiar habits. In the cooking club, his initial avoidance of Nanaba and Gelgar due to germophobia evolves into camaraderie after tasting Nanaba’s cooking. His silent, observant nature cements his consistent presence across the school’s social and extracurricular spheres.