Hayato Shinkai enjoyed a typical childhood and started competitive cycling in elementary school, quickly gaining recognition from numerous race wins. He attended Hadano Daiichi Middle School, joining its bicycle racing club where he became a feared and prominent competitor across the prefecture. A traumatic incident occurred during his second year: attempting to pass another rider on the left during a race, he collided with a mother rabbit. Though he fell, he recovered to win. Later discovering the deceased rabbit and her orphaned kit filled him with profound guilt for prioritizing victory over life. He adopted the kit, naming it Usakichi, and developed a crippling inability to pass on the left, instinctively braking due to fear of repeating the accident. This trauma led him to reject participating in that year's Inter High competition, causing disbelief among his team. Juichi Fukutomi, especially angered, confronted him physically before Shinkai explained. Fukutomi then reserved the number 4 bib for him, demanding his return the following year. Shinkai continued racing with Hakone Academy's cycling club, encouraging Touichirou Izumida to become a sprinter. Teammates Jinpachi Toudou and Yasutomo Arakita, learning of his trauma, offered to help him practice "passing on the right," with Fukutomi joining these sessions to support his recovery. Serving as Hakone's ace sprinter during the 41st Summer Inter High, Shinkai faced Kyoto Fushimi's Akira Midousuji at a sprint checkpoint on the second day. Midousuji, aware of Shinkai's trauma, deliberately provoked him by referencing the rabbit incident through lyrics of "Furusato" and veering far right to invite a left-side pass. Struggling with fear but recalling his team's support, Shinkai broke through his mental block. He activated his "Straight-Line Demon" persona—a frenzied, aggressive state contrasting his usual calm—and challenged Midousuji. Despite this, Midousuji narrowly won the checkpoint. Fukutomi later called upon Shinkai during the final sprint, trusting him to assist. Shinkai successfully launched Fukutomi with a powerful push, contributing directly to Hakone's victory that day. After graduation, Shinkai enrolled at Meiso University alongside Fukutomi. Before leaving, he joined Fukutomi, Toudou, and Arakita on a trip to a theme park. There, he engaged in playful competitions, briefly entering his "demon" state during a swan boat race against Arakita. Fukutomi explicitly thanked Shinkai during the trip for overcoming his trauma and returning as Hakone's "fastest and strongest ace sprinter in history." He attended the 42nd Inter High as a spectator, supporting Hakone Academy's team, which included his younger brother Yuuto Shinkai. Shinkai possesses reddish-brown hair, often depicted with blue streaks, and heavily-lidded eyes giving him a relaxed appearance. He frequently eats banana-chocolate flavored power bars and performs a signature "Bakyun" pose (a finger-gun gesture). Typically calm and placid, he harbors deep care and guilt over past actions. Under competitive pressure, particularly when unleashing his "Straight-Line Demon" state, his demeanor shifts to intensely focused and aggressive. He maintains close relationships, often using teammates' given names. His interests include reading detective novels and consuming Japanese snacks (notably eating entire cakes alone), while disliking cold weather. Usakichi remained at Hakone Academy after his graduation.