Nanako Watase is the diligent class representative at Minakami Private High School. She possesses long, wavy light brown hair reaching her back and warm brown eyes. Her kind nature is evident when she notices the new transfer student, Mizuki Hijiri, alone during a break and approaches her to introduce herself. Nanako formally asks Mizuki to be her friend, leading them to participate together in sporting activities.

In the early episodes, Nanako informs Mizuki about preparations for sports activities before encountering messages distributed on paper planes by the Hero Club. She later observes interactions involving Takumi Sekiya, accused of being behind the paper planes. While watching Mizuki play tennis, Nanako notes that no one was present when a nearby basketball hoop fell, contributing to discussions about unexplained incidents. She also expresses pleasure at seeing Kazuhiro Nakamura's parakeet, Faust, during a gathering at his house.

Her relationships include rejecting romantic advances from third-year student Takumi Sekiya, stating she needs time to consider having a boyfriend. To discourage Sekiya's continued attention, Nanako devises a plan to create a pretend romantic relationship. She selects Kazuhiro Nakamura for this role, noting his positive mood during an explanation about a sword's powers. She compliments his manga drawing skills and goes clothes shopping with him, demonstrating an ability to understand his idiosyncratic manner of speaking. During their pretense, they share an umbrella outside a mall and successfully convince Sekiya of their fabricated relationship.

Nanako expresses a liking for Vocaloid songs. Her given name, Nanako, incorporates meanings related to vegetables or greens ("菜") and child ("子"), while her surname, Watase, incorporates concepts of crossing or transit ("渡") and rapids ("瀬").