Hannes is the caretaker at Attack Junior High School, maintaining the campus environment and advising the Wall Cleanup Club, also known as the Wall Beautification Club. His responsibilities include guiding students, supervising club activities, and occasionally refereeing school events like dodgeball matches and Sports Day games.

He presents as an older man with a flustered demeanor, featuring light blond hair, a trimmed light mustache, and light brown eyes. In the anime adaptation, he wears an orange jumpsuit, while the manga depicts him in a dark tracksuit.

Generally laid-back and approachable, he frequently appears intoxicated, reeking of cheese and beer. Despite this, he shows genuine care for the school and its students, taking his advisory role seriously as his primary duty. His attempts to assist others—particularly Eren—often backfire comically due to drunkenness or clumsiness, such as malfunctioning equipment demos or accidental collisions.

Key moments include redirecting Eren and friends from the Titan section to the human side of the school. During a dodgeball match, he is knocked unconscious by a ball. In one incident, he crashes damaged omni-directional mobility gear while demonstrating it to Eren, colliding with Jean; he later blames his drunkenness and encourages Eren to persevere. When Titans chase students, he diverts their attention, enabling the group to reach the principal’s office before comically crashing through a window.

He shares a history with Eren Jaeger and Mikasa Ackermann, having saved them from the Colossal Titan during a lunch theft. He persistently supports Eren, striving to earn the student’s respect despite repeated failures. With Rico Brzenska, he maintains generally cordial ties as her club advisor, though she holds him responsible for damaging her gear equipment, straining their professional rapport.