Nikki Hanada is a character who begins as an antagonist before becoming a steadfast ally. Initially a member of the Tsukasa Empire, she is assigned to monitor Taiju Oki and Yuzuriha Ogawa. Her loyalty to the empire is genuine but ultimately secondary to a much deeper personal devotion.

Before the world was petrified, Nikki was a dedicated judo athlete, recruited for her large build and spending her school days training until she was covered in bruises. Her formidable physique and no-nonsense attitude earned her the nickname boss from her underclassmen. However, her tough exterior masks a surprisingly affectionate and sensitive nature. The defining experience of her youth was the bullying she endured, from which she found solace only in the music of her idol, the singer Lillian Weinberg. This emotional connection was so profound that she credits Lillian with saving her, making her a superfan with encyclopedic knowledge of the singer's career, from album sales to personal trivia.

In the stone world, Nikki's motivation is clear-cut: she is driven by an intense need to protect the legacy of Lillian Weinberg. This passion overrides any allegiance to Tsukasa. When she is tricked into a conversation with the mentalist Gen Asagiri, who is pretending to be Lillian, her deep knowledge immediately exposes the fraud. However, Senku Ishigami plays a genuine recording of Lillian's voice, and the moment she hears it, she is overcome with emotion. Realizing that the legendary singer is dead and that Senku possesses the last record of her singing, Nikki defects to the Kingdom of Science without hesitation. Her primary condition for switching sides is Senku's promise to protect that precious recording.

In the story, Nikki serves as a key member of the Kingdom of Science's power team, using her immense strength for construction, road building, and combat. She is a reliable fighter and laborer, often working alongside other physically strong characters like Kohaku and Magma. Her role is not just physical; after defecting, she becomes the coach for Gen Asagiri, helping him perfect his Lillian impression by teaching him subtle details like her southern accent. Her deep, almost obsessive knowledge of her idol becomes a strategic asset.

Nikki has several key relationships. Her bond with Senku is one of pragmatic trust and gratitude, as he holds the key to preserving Lillian's memory. She frequently has to physically restrain the impulsive Magma, often knocking him out to prevent his rash actions. Despite her intimidating strength, she reveals a surprisingly feminine side, showing interest in dyeing colorful fabrics and possessing delicate, girlish handwriting. This duality is a core part of her personality; she is proud of her power but can be flustered when others question her femininity.

Her development sees her evolve from a low-level sentry in an enemy empire to a trusted and integral member of the main crew. She participates in major expeditions, including the voyage on the Perseus to Treasure Island and the journey to America. In the America arc, she puts her self-taught English skills to use, acting as a negotiator with the American colony. Her resilience is tested in battle, as she is shot and later petrified, but she continues to fight for her new family.

Nikki's notable abilities are primarily physical. She is exceptionally strong and a skilled judo practitioner, capable of breaking cement with a hammer and taking down opponents with tackles. Beyond brute force, she possesses a highly trained ear and a fanatical attention to detail, allowing her to discern authentic vocal performances from imitations. She is also shown to have learned some English on her own, a skill born from her desire to understand Lillian's songs more deeply.