Nagisa Shirai is a supporting character in the anime and manga series Mermaid Melody Pichi Pichi Pitch. He is a young boy with short blue hair and gray eyes. He first appears in episode fifty-five of the anime and meets Hanon Hōshō in episode fifty-eight at an airport, where he retrieves a music sheet she had dropped. From that moment, Nagisa develops a strong crush on Hanon and persistently tries to win her affection. However, the music sheet he returned is a tune composed by Tarō Mitsuki, a music teacher for whom Hanon harbored a long-standing infatuation, making Nagisa's early efforts counterproductive.

Nagisa is two years younger than Hanon. In the manga, his personality is described as candid and childlike, while in the anime he retains a similar playful side but also exhibits a more energetic and occasionally impulsive temperament. Despite Hanon's initial irritation with his advances, Nagisa remains resilient and devoted. Over time, his steadfastness leads Hanon to gradually warm to him. He is unaware that Hanon is a mermaid, as he has never seen her true form.

In the story, Nagisa serves as a human counterpart to Hanon and becomes her eventual love interest. His main motivation is to earn Hanon's genuine affection and to be close to her. Throughout the series, his character develops from an overly eager admirer into a more earnest and supportive partner. His role is primarily that of a romantic interest, and he does not possess any supernatural abilities. In the manga, Hanon eventually falls in love with Nagisa, and they begin a relationship by the end of the series. In the anime, they also become a couple in the second season, Mermaid Melody Pichi Pichi Pitch Pure.