Haruka Nanase is a first-year student at Iwatobi Middle School, navigating a new chapter of his life after the disbandment of his elementary school swim club. This period is marked by significant change, as his childhood friend Makoto Tachibana is placed in a different class for the first time, leaving Haruka to face the unfamiliar environment of junior high on his own. He lives with his family, which includes his mother, and at one point in the story, her temporary absence provides an opportunity for his new friends to bond with him outside of the pool.

Haruka is defined by a quiet, curt personality and a general disinterest in dealing with others, which can make him seem aloof. At the core of his character is a profound and special connection to water. This connection is not merely about competition or victory; it is an intrinsic, almost personal feeling. His primary motivation initially is not to win races but to simply experience the act of swimming, especially the freestyle stroke, which is his forte. Despite his withdrawn nature, his skilled and elegant swimming has a captivating effect on those who witness it.

In the story of High Speed! -Free! Starting Days-, Haruka is reluctantly pulled back into competitive swimming by the enthusiastic Asahi Shiina and the persistent Iwatobi swim team captain, Natsuya Kirishima. Although he has little initial interest in joining, a practice race in which he ties with Natsuya secures his right to swim only freestyle on the team. His role evolves from a solitary swimmer to a crucial member of a medley relay team, a format that requires cooperation and trust. This challenges his preference for solo freestyle swimming and forces him to engage with teammates who each have their own struggles and reasons for swimming.

Haruka’s key relationships are central to the film’s narrative. His bond with Makoto Tachibana, his oldest and closest friend, is complex. Makoto usually acts as a stabilizing force, understanding Haruka’s unspoken thoughts and speaking for him. When Makoto begins to doubt his own reasons for swimming, the distance that grows between them forces Haruka to find his own voice, directly confronting Makoto to restore their friendship. With Asahi Shiina and Ikuya Kirishima, Haruka forms the core of the new relay team. Ikuya, the quiet younger brother of the captain, initially resents Haruka for the attention his brother gives him and begins to imitate Haruka’s swimming. As they live together through a team defeat and personal crises, including Ikuya threatening to quit and Haruka fainting from exhaustion, they forge a bond of genuine friendship and mutual support. Natsuya Kirishima, the team captain, serves as a coach and rival who recognizes Haruka's talent and pushes him to be part of a team. The shadow of Rin Matsuoka, Haruka’s rival from elementary school, also looms over the story, as Rin’s friend Sousuke Yamazaki challenges Haruka to a race and eventually delivers a letter from Rin that reveals his struggles in Australia. Receiving this letter gives Haruka a new sense of purpose and a desire to become a better swimmer with his team.

Throughout the film, Haruka undergoes significant development. He transitions from a boy who swims alone, disconnected from the competitive goals of others, to someone who understands the value of teamwork. He begins to articulate his feelings more directly, first in his confrontation with Makoto and later in his commitment to his new friends. By overcoming individual and collective challenges to win the prefectural tournament medley relay, Haruka learns that his passion for swimming can coexist with—and even be strengthened by—the bonds he forms with a team. The story concludes with him feeling determined, a departure from his earlier passive and solitary approach to the sport. His notable ability is an exceptional talent for freestyle swimming, a stroke he swims with such grace and skill that it captivates onlookers and motivates his rivals.