Hiroki Tsujiai is a male student at the same middle school as the story's protagonist, Ayu Tateishi. He is known for his athleticism, holding the position of president of the boys' tennis club. In terms of his physical appearance, he is a teenager with black hair cut to ear length and brown eyes.

His role in the narrative is primarily defined by his close friendship with Tetsushi Kaji. The two are inseparable classmates and best friends, and Hiroki is frequently seen alongside him. Despite being Tetsushi's best friend, his behavior initially puts him at odds with Ayu. Ayu is in love with Tetsushi, but she considers Hiroki to be a hindrance who often unintentionally sabotages her attempts to get closer to her crush.

Beyond his athletic commitment to tennis, Hiroki has a more relaxed and slightly extravagant side. He shares a particular interest in manga, and he is a fan of the same series as Nina Sakura, the young witch who is a central character. This shared hobby forms a basis for his growing relationship with Nina. Over the course of the story, his feelings for her develop, and he eventually begins a romantic relationship with her. In this context, he becomes aware that Nina is a magical girl from another world, making him a confidant to her secret.