Natsuko Honda is a high school student who transfers to Kure Academy and becomes a classmate of Nobuaki Kanazawa. She is the twin sister of Chiemi Honda and the daughter of Kazunari Honda. Her father named her after his first love and cousin, a woman who died in the very first King's Game. Natsuko herself survived a previous King's Game at her former school, an experience that fundamentally altered her personality and left her with a deep-seated drive to survive at any cost.

On the surface, Natsuko presents herself as cheerful, outgoing, and friendly. She is the first person to welcome Nobuaki when he transfers, and she is known among her classmates for being popular and sociable. She is also an accomplished runner, serving as the anchor of the school marathon team. This bubbly exterior, however, is a deliberate facade. The trauma of her past game has twisted her true character, revealing a calculating, manipulative, and ruthless individual. She is highly intelligent and perceptive, able to quickly read others and exploit their weaknesses for her own advantage. Her primary motivation is self-preservation; she believes that the only way to survive the King's Game is to prioritize her own life above all others, and she is willing to sacrifice, deceive, and turn against anyone who threatens her safety.

In the story, Natsuko initially appears to be a helpful and supportive friend, but she soon exposes her dark nature. She deliberately attempts to sow distrust among the class, accusing Nobuaki of attempted rape to isolate him and turn his classmates against him. She tries to coerce another student into sending a text that would cause Nobuaki's death, and she uses her intelligence to manipulate situations to her benefit, often acting in direct opposition to Nobuaki's goal of uniting the class. As the game progresses, her desperation and aggression escalate. She eventually engages in a final violent confrontation with Nobuaki, resulting in his death. Her relationship with her twin sister Chiemi is complex, and her bond with her father remains strained due to the painful legacy of the original King's Game. Throughout the series, Natsuko shows little genuine growth or redemption; she remains relentlessly focused on her own survival, and her development underscores the tragic and corrupting influence of the King's Game itself. Her notable abilities include physical strength and quick reflexes, as demonstrated when she overpowers a male classmate who attempts to assault her, as well as a sharp intellect that makes her a formidable player in the deadly game.