He begins as an unnamed male high school student who chooses solitude and reading over social interaction. Consciously avoiding personal connections due to a belief that relationships inevitably cause mutual hurt, he remains detached and emotionally reserved. His plain appearance features jet-black unstyled hair and standard school attire: a pale beige blazer, white shirt, blue necktie, black dress pants, and dark brown loafers. Outside school, he dons simple, nondescript clothing to remain unnoticed.

His life shifts upon discovering a handwritten journal titled "Living with Dying" in a hospital, belonging to his popular and outgoing classmate, Sakura Yamauchi. The journal reveals her terminal pancreatic illness. After Sakura confronts him about finding it and requests secrecy, he reluctantly agrees to spend time with her, initiating their unconventional relationship. Sakura actively involves him in bucket list activities, including dining excursions and a Fukuoka trip. Sharing a hotel room during the trip, he discovers her extensive medication, forcing him to confront her condition's physical reality. Their interactions include playing truth-or-dare, where Sakura asks probing personal questions and he reluctantly engages in physical dares, such as carrying her to bed.

Subtle changes emerge in his behavior as their time together progresses. He starts showing concern for Sakura's well-being, particularly during her hospitalization. He engineers her temporary escape to view fireworks, where he directly wishes for her survival. During an emotionally charged home visit, Sakura teases him about romantic feelings, triggering an outburst: he physically pins her down before abruptly leaving, revealing emerging emotional volatility. His growing attachment leaves him vulnerable, and when Sakura dies unexpectedly from random violence before a planned meeting, he experiences intense grief. He avoids her funeral and isolates himself.

His subsequent actions show significant development. He visits Sakura's parents and receives her journal as per her final wishes. Through it, he learns her perspective on their relationship: her understanding of his emotional isolation and her reciprocated feelings. During this visit, his full name, Haruki Shiga, is revealed, symbolizing a transition to self-acknowledgment. Sakura's mother explains the connection between their names—Haruki meaning "spring trees" and Sakura meaning "cherry blossom"—highlighting a thematic link to spring and renewal. Fulfilling Sakura's request, he locates her best friend Kyoko, shares the journal's contents, and overcomes his avoidance of emotional confrontation. He directly asks Kyoko to be his friend, explicitly acting on Sakura's influence to open himself to human connection. A year later, he and Kyoko visit Sakura's grave together, confirming his sustained commitment to relationships and living fully, embodying Sakura's inspired change.