Hiromi Higa, who is known by the alias Shadow, is a 24-year-old man who lives a life divided between the daytime and the nighttime. He was born on September 14th, making his zodiac sign Virgo, and has a blood type of A. Physically, he is tall and muscular, standing at 178 centimeters tall and weighing 73 kilograms. He has distinct orange hair styled into a short mohawk, dark eyes, and a muscular build. A tattoo of the stylized letters SD is visible on his left bicep. During the day, Hiromi works a perfectly normal job at a flower shop. In this role, he is known for being friendly, kind, and very serviceable to the customers, always maintaining a positive and helpful attitude. This gentle personality is his true, everyday nature. However, when night falls and he enters the secret, high-stakes skateboarding race called S, he transforms into his alter ego, Shadow. In stark contrast to his daytime self, Shadow adopts an arrogant, rude, and thuggish persona, dressing in a flamboyant heavy rocker style with a sleeveless black top, a torn cape, and wearing white makeup with green eyeliner and purple lipstick. He is a showman who thrives on the attention of the crowd and presents himself as a bombastic anti-hero. His motivations for this dual life are tied to his job; the frustrations of his day, such as a customer complaining about flowers or a canceled order, are channeled into his performances as Shadow, where he can be rowdy and release his stress. His goal in the races is not just to win but to put on a good show with his signature dirty tricks and fireworks.

Within the story, Hiromi is initially presented as a kind of antagonist who picks fights with the main characters Reki and Langa, but he quickly evolves into a much more complex figure and a member of the protagonists' circle. Despite his loud protests and tough-guy exterior, he becomes something of a mentor and older brother figure to the younger skaters, especially Miya. He is often seen giving them rides in his car and taking them out to eat, acting as a reluctant but responsible caretaker. This is highlighted in the Extra Part OVA, which features a segment titled Hiromi, Do Your Best! that focuses on his noble and hardworking personality at the flower shop. A key aspect of his development is his growing alliance with Reki, Langa, and the others. While he continues his morally dubious skating antics, he is one of the first to warn the group about the truly dangerous nature of the legendary skater Adam, making it clear that despite his own dirty tricks, he sees a fundamental difference between himself and that antagonist. In terms of notable abilities, Shadow is a regular-footed skateboarder, meaning he prefers to ride with his left foot at the front of the board. His signature skating style is characterized by underhanded tactics, such as throwing firecrackers or using various distractions to unsettle his opponents, cementing his reputation as a tricky and unpredictable racer.