Haruki Shiga presents as a typical high school student with a plain appearance and perpetually listless expression. His jet-black hair falls naturally, unstyled, contributing to his unremarkable look. His standard-issue school uniform consists of a pale beige blazer with brown details, a white shirt, a blue necktie, black dress pants, and dark brown loafers. Outside school, he favors simple, non-distinctive clothing like plain-colored t-shirts. Some sources note his green eyes.

Initially introverted, anti-social, and emotionally reserved, Haruki actively avoids social interactions, maintains no friendships, and expresses minimal emotion, finding solace in solitude and reading. This detachment stems from a deep fear of relationships causing mutual hurt. His life shifts dramatically after discovering the journal "Living with Dying," belonging to his terminally ill classmate Sakura Yamauchi, in a hospital. Sakura recruits him to help fulfill her bucket list, valuing his honest, pity-free perspective.

Key interactions challenge Haruki's barriers. During a trip, Sakura corners him into playing truth-or-dare, forcing vulnerable moments like carrying her to bed and sharing a bed. A pivotal conflict erupts at Sakura's home when she jokes about kissing him; Haruki reacts violently, pinning her down before fleeing in immediate regret. This leads to a confrontation where Sakura's jealous ex-boyfriend, Takahiro, assaults Haruki. Sakura physically defends Haruki, revealing her deeper care.

Through these experiences, Haruki gradually opens up, learning to express concern and affection. He confesses his desire for Sakura to live during a hospital visit and shares a heartfelt moment watching fireworks. Her unexpected death—from a random stabbing, not her illness—shatters him. He isolates himself, skips her funeral, and drowns in grief. Later, visiting Sakura's parents, he receives her journal and learns her true feelings: she cherished their connection, metaphorically "eating his pancreas" to carry his strength inside her. Her final request urges him to reconcile with her best friend, Kyoko.

A year after Sakura's death, Haruki shows significant growth. He befriends Kyoko, visits Sakura's grave, and embraces social connections. His name, meaning "spring trees," complements Sakura's ("cherry blossom"), symbolizing their intertwined fates. In the sequel novel "To My Father and to Someone in My Memories," Haruki is revealed as a father to Fuyumi, his life continuously shaped by Sakura's influence. His journey concludes with a commitment to living fully, honoring Sakura by maintaining relationships and cherishing memories.