Mizuho Nanba works as an assistant director at MRS radio station alongside Kanetsugu Mato. She sports brown hair and brown eyes, typically dressed in a green jacket over a white shirt and a long pink dress. Mizuho first encounters Minare Koda when Minare bursts into the radio studio during a live broadcast. After this incident, Mizuho joins colleagues for dinner at Minare's workplace, where Minare receives a job offer from the station.

Once Minare accepts the radio position, Mizuho notices her intention to sleep in the studio storage room and offers her temporary accommodation at her own apartment. During their time living together, Mizuho discusses her appreciation for radio work with Minare and clarifies that the "8 seconds rule" Kanetsugu Mato previously cited was entirely fabricated. This revelation inspires Minare to devise a strategy using listener complaints to boost engagement, a plan Mizuho finds amusing. They spend evenings listening to the station's night programming together.

Mizuho exhibits a generally kind and supportive nature, extending help to others like providing shelter to Minare during her housing instability. She maintains a mature outlook most of the time but displays occasional naivety regarding relationships involving Madoka Chishiro and Katsumi Kureko. Her background includes no prior romantic relationships or experience with boyfriends.

Within the radio station, sound engineer Ryusuke Komoto harbors unrequited feelings for her. Mizuho also undertakes professional assignments outside the studio, including a trip to Wassamu where she, Minare, and Kureko become hostages of the School of Wave Wisdom cult. The group is coerced into producing programming for the cult during this ordeal.

Her name carries specific meanings: "Mizuho" combines the characters for "auspicious" (瑞) and "grain" (穂), while "Nanba" translates to "south" (南) and "wave" (波).