Nano Shinonome is an android created by the child inventor Hakase, residing at the Shinonome Laboratory. Though only one to two years old chronologically, she possesses the physical appearance and general intelligence of an average teenage girl. A large wind-up key protrudes from her back, installed by Hakase for aesthetic reasons despite Nano's repeated requests for its removal due to the inconvenience and fear it exposes her robotic nature.

Her primary role involves managing household responsibilities at the Shinonome Laboratory, encompassing cooking, cleaning, grocery shopping, and caring for Hakase and Sakamoto, a talking cat gifted with speech via a scarf crafted by Hakase. Nano exhibits a maternal and responsible demeanor within the household, frequently lecturing Hakase about behavior or unhealthy snacks. However, her personality shifts in social settings outside the home; she becomes reserved, shy, and deeply insecure about her android identity, harboring a strong desire to be perceived as an ordinary human girl and experience a typical school life. This internal conflict drives much of her development.

Built to serve as a companion and caretaker, Nano endures frequent modifications by Hakase without consent. Hakase adds impractical or whimsical functions causing Nano distress when they activate unexpectedly. These include a rollcake dispenser in her left arm, a sweet bun dispenser in her forehead, a cuckoo clock, rocket propulsion in her limbs, super-strength, flight capabilities via foot rockets, and concealed storage in her toes. Nano remains largely unaware of these functions until they activate, often in socially compromising situations. She typically avoids utilizing her robotic abilities, preferring conventional human methods—like using an abacus for math problems—to maintain a facade of normalcy. The key on her back, while mostly non-functional, can launch her big toe like a rocket when manually turned.

A significant milestone occurs when Hakase permits Nano to attend Tokisadame High School. Initially, Nano struggles with anxiety over potential exposure as an android, particularly around classmates Yuuko, Mio, and Mai. She fabricates obvious excuses about the key and mechanical mishaps, fearing rejection. However, Yuuko's declaration that "Nano is Nano" after discovering her true nature—and the group's subsequent indifference to her robotic traits—marks a turning point. This acceptance allows Nano to gradually embrace her identity. By the series' conclusion, when Hakase offers to remove the key, Nano chooses to retain it as an integral part of herself, symbolizing her self-acceptance.

Her relationships are central to her growth. With Hakase, Nano balances frustration over unwanted modifications and broken promises with genuine care and loyalty. She views Sakamoto as an elder deserving respect, though she occasionally treats him as an ordinary cat, much to his annoyance. Her friendships with Yuuko, Mio, and Mai provide social belonging, with Yuuko's enthusiastic acceptance proving particularly pivotal in easing Nano's fears. Despite initial awkwardness, these bonds reinforce her realization that her value is not diminished by her artificial origins.

Nano's development culminates in a more relaxed and self-assured outlook. She transitions from concealing her true self to acknowledging her robotic traits as inseparable aspects of her identity, finding contentment in ordinary experiences like attending school and building friendships. Her journey emphasizes themes of belonging and self-acceptance, framed within the juxtaposition of her mechanical existence and human-like aspirations.