Nikki Hanada, also referred to as Nicki, is a modern-era survivor who was revived from the petrification by Tsukasa Shishio and initially served as a member of the Tsukasa Empire. She later defects from the empire and joins the Kingdom of Science, becoming a reliable and physically powerful member of the group. Her background reveals that during her childhood and school years, she was bullied for her masculine appearance and build, which caused her significant distress. She found solace and emotional salvation in the music of the singer Lillian Weinberg, becoming a devoted superfan who memorized extensive trivia about Lillian's career and southern accent. This deep personal connection drives her primary motivation: protecting the final surviving copy of Lillian Weinberg's song, which she believes is irreplaceable. When she hears Senku Ishigami play the song through a communication device, she immediately switches her allegiance to the Kingdom of Science because Senku promises to preserve Lillian's legacy.

Nikki has a straightforward, strict, and reliable personality. She is disciplined and often takes on an unofficial disciplinary role within the Kingdom of Science, frequently restraining impulsive or violent members like Magma and Yo from acting rashly. Despite her physical strength and tough demeanor, she is somewhat self-conscious about appearing too rugged or lacking traditional femininity. Her sense of duty is strong, and she earns respect from her peers through her dependability and protective nature. In the New World arc, as the Kingdom of Science travels to North America, Nikki becomes a crucial figure in the construction and settlement of Corn City, handling heavy physical labor and helping to establish the community.

Her key relationships include Tsukasa, who revived her but whose empire she leaves behind; Senku, for whom she develops a strong admiration and even a romantic interest after he honors Lillian's music; Gen Asagiri, whom she coaches on how to imitate Lillian's voice; and Magma, whom she frequently keeps in check. She also maintains a big-sister-like rapport with many younger or less disciplined members. Her notable abilities stem from her years of judo training in secondary school, where she was recruited for her muscular build and became known for her toughness, earning the nickname "boss" among underclassmen. She exhibits immense physical strength, capable of breaking concrete with a hammer and carrying large men like Magma. She is also a member of the Power Team, assigned to tasks requiring brute force. Over the course of the story, she develops from a loyal soldier of Tsukasa's empire into a dedicated and integral worker for the Kingdom of Science, embracing a more cooperative and constructive role in rebuilding civilization.