She is a human-klaxo sapien hybrid and a member of the APE Special Forces, primarily fighting on the front lines. As an elite Parasite with the codename "", also known as the "Partner Killer", Zero Two had always accepted solitude due to her horns and Klaxosaur blood. Despite having no regard for human life or her own and being accustomed to fighting solo, she took interest in Hiro and offered an opportunity to pilot with her, making him her new partner and "darling". Over time, Zero Two developed romantic feelings towards Hiro. However, her determination to become fully human, along with Hiro's apparent fear of her increasing feral characteristics resulted in the couple briefly drifting apart. When their relationship as partners began to take a turn and slowly crumble, they realized their forgotten memories of their meeting during childhood.