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Heero Yuy, sometimes referenced as Heero Yui, was born to OZ agents Aoi Clark and Odin Lowe in the A.C. 170s. Following Odin Lowe's assassination of pacifist leader Heero Yuy and the end of their relationship, the boy lived with his mother and stepfather Seis Clark until their deaths. Odin Lowe subsequently assumed guardianship, training him in survival and combat. On colony L3-X18999 in A.C. 188, during an assassination mission, the boy witnessed Odin Lowe's death; Odin's final words urged him to "live by his emotions" and declared "one foolish person can change the world," a philosophy pivotal to his future.

Discovered adrift in the colonies by Doctor J, he underwent genetic manipulation and rigorous training, enhancing his physical control, discipline, and reasoning to become an exceptional operative skilled in close combat, hacking, and mobile suit piloting. During an A.C. 194 mission destroying an Alliance base, he accidentally killed a young girl and her dog, an event instilling profound self-loathing and remorse. Dekim Barton then ordered Doctor J to suppress his emotions to forge a more effective weapon, though the depth of this retraining remains undefined.

Chosen in A.C. 195 to pilot the Wing Gundam for Operation Meteor, Doctor J assigned him the codename "Heero Yuy" as a tribute to the assassinated leader. He rejected Dekim Barton's genocidal plan to drop a colony on Earth, targeting only OZ military facilities instead. Initially cold, stoic, and mission-obsessed, he frequently made suicidal decisions, like triggering his flight suit's self-destruct after an Earth crash. He repeatedly threatened death to Relena Darlian—who discovered his identity—to push her from danger, yet saved her life multiple times when OZ targeted her.

Throughout the conflict, he formed reluctant bonds with fellow Gundam pilots, notably Duo Maxwell, gradually acknowledging his suppressed emotions. Utilizing the ZERO system, which amplified his consciousness and combat prowess, forced him to confront psychological trauma and philosophical contradictions. Despite declaring "peace is but a consequence of war," his actions evolved to protect civilians and align with Relena's pacifist ideals.

In the Endless Waltz conflict (A.C. 196), he confronted lingering guilt over the girl's death and rejected Gundams as tools of war. After thwarting Mariemaia Khushrenada's rebellion, he sent the Wing Gundam Zero into the sun alongside other Gundams, symbolizing his commitment to peace. The Frozen Teardrop sequel reveals decades later he underwent cryogenic preservation, eventually proposing to Relena Darlian. By the epilogue, he lived peacefully with her on Mars, discussing marriage and family.