Hannes served as a unit captain in the Garrison Regiment, initially stationed in Shiganshina District. He maintained a long-standing friendship with Grisha and Carla Yeager, originating when Hannes guarded Grisha after Keith Shadis found him beyond Wall Maria. Intoxicated during duty, Hannes speculated Grisha had wandered outside drunk. Grisha later cured a plague in Shiganshina that struck Hannes' wife, creating a lasting debt Hannes vowed to repay. Their bond deepened when he attended Grisha and Carla's wedding. Before Wall Maria fell, Hannes displayed a carefree, irresponsible nature, often drinking on duty and avoiding responsibilities. He dismissed intervening in Eren Yeager’s childhood fight, leaving it to Mikasa Ackermann and Armin Arlert. He similarly brushed off Eren’s Titan-related fears, insisting the walls were impenetrable. His character transformed during Shiganshina’s Titan attack. Tasked with rescuing Eren and Mikasa, he discovered Carla trapped under debris. Confronted by a Smiling Titan, Hannes froze in fear, abandoned Carla, and fled with the children. He tearfully confessed his cowardice to Eren, acknowledging his failure stemmed from lacking courage. This event haunted him, fueling a desire for redemption. Over the next five years, Hannes dedicated himself to military service, rising to Garrison captain. He became disciplined and responsible, shedding his earlier complacency. Reuniting with Eren, Mikasa, and Armin after their Cadet Corps graduation, he apologized for failing Carla—though Eren absolved him. Hannes continued watching over them, expressing relief spotting Eren with Commander Pyxis during Trost’s invasion. During the mission to reclaim Wall Rose in the Clash of the Titans arc, Hannes prioritized locating Eren after the Colossal Titan’s reappearance. When Reiner Braun (Armored Titan) and Bertholdt Hoover (Colossal Titan) kidnapped Eren, Hannes helped coordinate a joint Survey Corps-Military Police rescue. He vowed to retrieve Eren at any cost, urging his unit onward. In the forest outside Wall Rose, Hannes’ unit found the kidnappers but failed to capture them. As Titans overran the ensuing battle, Eren and Mikasa were thrown from their horse. Hannes intervened to shield them from an approaching Titan, recognizing it as the Smiling Titan that killed Carla. Determined to atone, he engaged it directly. Despite a fierce effort, the Titan overpowered and killed him before Eren and Mikasa’s eyes. His death deeply impacted Eren, Mikasa, and Armin, who later mourned him and reflected on their shared past. Eren acknowledged Hannes among lives lost to the Titan power conflict. On the eve of reclaiming Wall Maria, the trio briefly mistook another Garrison soldier for Hannes, triggering memories of his sacrifice. In the "Lost Girls" OVA, Hannes assisted Grisha by investigating an assault on young Eren. Using witness accounts, he identified the attackers. They died in a carriage accident before facing justice, and Hannes reported this outcome to Grisha with sketches of the men.