Kaji's best friend, who is often seen together with him. He is the head of the boys' tennis club. He is a fan of many manga and has similar taste with Nina. He is very steadfast and has a good sense of self-worth. Hiroki is in love with Nina, and he and Nina start to go out with each other mid-way through the manga. They continue to date up to the end of the series.
He is generally quite calm and mature and quite liked by his peers. He is rarely seen without his best friend by his side. Nina tells him of her magical powers later in the series
In the manga and anime, Hiroki is mostly a very laid back person.