Hiroto Suwa is one of the main characters in the story and a member of the close-knit group of friends at the center of the narrative. He is a student and an avid member of the school's soccer team. Physically, he is depicted as tall and muscular, standing at 185cm. He has a friendly and easygoing personality, often appearing extroverted and happy, which makes him approachable to others. This open nature is highlighted when he is the first person to welcome the transfer student Kakeru Naruse into their circle.

While he presents a cheerful and sometimes clueless exterior, Suwa possesses a deep well of kindness and selflessness. He occasionally displays a gluttonous streak and a good sense of humor, and he is not afraid to do embarrassing things for the sake of his friends, such as giving another boy flowers or begging him to join a sports club. His primary motivation is the happiness and well-being of those he cares about, even at the expense of his own desires. He is willing to put the feelings of Naho Takamiya and Kakeru before his own to give them a chance at happiness together.

In the story, Suwa serves as a pivotal figure in the central love triangle and the group's collective effort to change the future. He also receives a letter from his future self, which contains knowledge of the regrets and events to come, including the revelation that he marries Naho in the original timeline where Kakeru dies. Despite his own romantic feelings for Naho, which he harbors from the beginning, Suwa is determined to follow his future self's instructions to support Naho and Kakeru's relationship, effectively sacrificing his own chance at a future with her. His role becomes that of a supporter and at times, a facilitator, pushing the two together to prevent the tragic outcome of Kakeru's suicide.

Suwa's key relationships are with Naho and Kakeru. His bond with Kakeru is that of a close and loyal friend, which is why he chooses to step aside romantically. His relationship with Naho is complex, founded on a long-standing friendship and his unspoken love for her. In the original future timeline, after Kakeru's death, Suwa and Naho eventually marry and have a child, though his guilt over the circumstances of their union is a significant burden. In that darker future, he uses ideas that Kakeru had shared with him to try and mend Naho's broken heart, a fact that later makes him see himself as having taken advantage of the situation.

Suwa undergoes significant development as he grapples with the instructions from his future self. His internal conflict involves his love for Naho and his desire to be a good person versus the guilt of the future he came from, where he feels he did not do enough to help Kakeru. He decides not to confess his feelings to Naho on New Year's Eve, the event that triggered their relationship in the original timeline, choosing instead to simply comfort her and push her toward Kakeru. This act of self-denial shapes his character arc, transforming him from a potential romantic rival into a crucial, self-sacrificing supporter. His development highlights his altruism and his resolve to live without the deep regrets of his future self, who believed he had been a "dogged nice guy" for moving in after Kakeru's death.

As a member of the soccer team, Suwa is athletic and physically capable, though the story does not focus on any special abilities beyond his normal human traits of loyalty, empathy, and determination.