Hiyori Minagi, originally named Hiyori Yamakawa, is a first-year high school student. Her life shifted significantly three years before the main events when her father died from cancer. During her childhood, her father introduced her to fly fishing, an activity she developed a strong affinity for. She often visited the seaside to fish, finding it a source of solace while coping with her loss. Following her mother's decision to remarry, Hiyori reluctantly prepared to meet her new stepfamily. Seeking comfort, she went to her usual fishing spot. There, she encountered Koharu Minagi, an energetic girl who expressed interest in fly fishing. This meeting proved pivotal, revealing Koharu as the daughter of her mother's fiancé, making them stepsisters. After the marriage, Hiyori's surname changed to Minagi, though her childhood friend Koi Yoshinaga still called her "Yamahi," a blend of her former surname and first name.

Hiyori possesses a reserved and introverted personality, often displaying shyness in social situations. Clumsiness plagues her in most activities, yet she demonstrates natural skill in fishing, a legacy of her father's teachings. Her shyness manifests as blushing, stuttering, and avoiding eye contact, particularly during early interactions with Koharu. She hesitated to use Koharu's first name, struggled to express gratitude directly, and became flustered when accepting invitations. Gradually, she opened up through shared experiences, especially fishing. She harbors a distaste for handling live bait or insects, stemming from her exclusive focus on fly fishing.

Her relationship with Koharu forms a central part of her development. Initially anxious about their new family dynamic, Hiyori found common ground through fishing, eventually becoming Koharu's mentor. Their bond deepened through recurring activities: they held hands during resonant moments atop a lighthouse and while entering school together; they slept side by side, leading to promises of future trips; and they collaborated on cooking, with Hiyori learning to prepare dishes beyond sashimi to suit Koharu's preferences. These interactions helped Hiyori become more communicative and less withdrawn. Her protectiveness and attentiveness toward Koharu grew, evidenced by her need to "keep eyes on her" and her view of Koharu as "unique."

Outside her immediate family, Hiyori maintained a longstanding friendship with Koi Yoshinaga, whom she knew since kindergarten. She expressed genuine appreciation for their fishing trips, sometimes causing Koi visible shyness. The ocean served as a therapeutic space for her, aligning with her calm and reflective nature. Her character arc emphasizes healing from grief through familial bonds and shared passions, illustrating how fishing became a conduit for emotional connection and personal growth.