Yū Himura endured profound childhood loss when a major earthquake destroyed his home, triggering a fire that killed his parents and younger sister Akane. He carried deep guilt, believing he failed to save Akane after hearing her calls for help during his escape. Survivor's guilt manifested in recurring nightmares. His sole memento was a broken wristwatch, previously gifted to Akane, kept as a tangible reminder of his loss and regret.

After the disaster, he lived in a church orphanage. There, he met six-year-old Yuuko Amamiya, who resembled Akane. Yuuko frequently followed him, calling him "onii-chan." Himura rejected her companionship, burdened by unresolved trauma and grief tied to his sister. His harsh rejection, explicitly stating he did not want a sister, occurred just before Yuuko's adoption by another family, leading to separation. He eventually forgot her entirely.

Years later, a hardship scholarship brought him to Otobane Academy. He formed close friendships with classmates Kuze Shuichi and Nagi Hirono, maintaining a quiet, reserved demeanor. At seventeen, he encountered Yuuko again when she enrolled at the same school. Initially unaware of their past, he felt inexplicably irritated by her presence. Upon recognizing her, she confessed enduring feelings and revealed abuse suffered at the hands of her adopted brother. This disclosure ignited his protective instincts, and he helped her flee their abusive home. They lived together briefly in a small apartment until her brother located them and forcibly returned her. Yuuko only rejoined Himura permanently after her abuser's death.

Their renewed relationship ended abruptly when Yuuko died in a traffic accident on Christmas Eve. A car struck her while retrieving a ball in the street. She succumbed alone, leaning against a tree while singing a song she had taught a young girl named Miki (later known as Mizuki Hayama). Himura arrived with Miki to discover Yuuko's body, compounding his existing emotional burdens.

As an adult, he pursued architecture, earning a degree and relocating to Australia. He played a key role in designing Otowa, Australia—an exact replica of the original Japanese town destroyed in his youth—as a tribute to Yuuko's memory and influence. He became the legal guardian of Chihiro Shindou, displaying fierce protectiveness over her well-being due to understanding her personal struggles. He frequently spent time in the local church, openly admitting he was not a Christian but was "waiting for someone." This behavior often led others to mistake him for a priest or counselor. He occasionally offered advice to visitors, though he typically avoided full transparency, responding vaguely or evasively to direct questions.

His adult personality reflected quiet thoughtfulness and emotional reserve, shaped by enduring sadness over past losses. He possessed skill in drawing landscapes but disliked admitting this artistic ability. A recurring trait was his dismissal of miracles, famously stating: "Miracles don't exist in this world. There are only inevitabilities, accidents, and the choices you make."

At twenty-nine, he received news from Nagi Hirono that her brother encountered a spectral manifestation of Yuuko in the original Otowa church in Japan. He immediately traveled to Japan and, after twelve years of waiting, reunited with Yuuko's spirit on Christmas. During this encounter, he presented her with the bouquet he never had the chance to give her before her death. He revealed he had moved away from atheism, though he remained agnostic. Yuuko encouraged him to seek happiness and stop dwelling on the past before disappearing, leaving behind a single white feather. This reunion provided a measure of closure. He later met Mizuki Hayama, now grown, who revealed her childhood identity as Miki and connection to Yuuko. Together, they viewed the realized town of Otowa, Australia—a testament to his architectural vision and a symbol of a hopeful future inspired by those he loved.