Hiroki Tsujiai attends middle school alongside Ayu Tateishi, Nina Sakura, and Tetsushi Kaji. He leads as captain of the boys' tennis club and possesses a slightly extravagant personality with a stubborn streak. His actions frequently disrupt Ayu Tateishi's attempts to get closer to her crush, Tetsushi Kaji, fueling her resentment. Hiroki becomes the third person to learn Nina Sakura's secret as a witch from the Magic Kingdom after witnessing her cat, Rio, transform into a human. This discovery deepens his connection to Nina. He develops strong affection for her, sharing her enthusiasm for manga and introducing her to series like Doraemon. Hiroki also assists Nina academically, particularly with her challenging English studies. In the manga continuity, Hiroki and Nina's relationship evolves into a romantic one midway through the series, and they remain a couple at the conclusion. Hiroki demonstrates loyalty and protectiveness, especially towards Nina. His role expands significantly when other characters from the Magic Kingdom arrive, drawing him deeper into the narrative's magical conflicts. The anime adaptation presents a different focus, centering on Nina's quest to collect five Holy Stones to qualify for marrying the Magic Kingdom's prince. Hiroki still discovers Nina's secret and actively supports her during this stone-collection mission. A pivotal moment occurs when Hiroki and Nina jointly trigger the release of the final Holy Stone while reaching out to each other within a dark zone. Hiroki maintains a close, consistent friendship with Tetsushi Kaji throughout both the manga and anime, often seen together and offering mutual support. His character displays analytical tendencies and a preference for independent work, yet he remains engaged within his small circle of friends. The resolution of his romantic arc with Nina concludes positively in the manga, while the anime emphasizes his unwavering presence as a supportive figure amidst the magical challenges.