Kotaro Nanbara is a wealthy second-year engineering student at Ochanomizu Industrial University, attending the same program as Kazuya Saotome. His background is marked by affluence; he lives in a mansion and drives a personalized car. Nanbara is defined by an eccentric, egomaniacal personality, often acting with high energy and a dramatic flair. He is deeply jealous of Kazuya, who consistently outperforms him in projects despite being less well-off, and he takes pleasure in seeing his rival struggle. His primary motivation is to prove his own superiority, which leads him to create a computer virus intended to destroy Kazuya's work. This virus, however, inadvertently causes Kazuya to order the cyberdoll May. Throughout the story, Nanbara acts as an antagonistic force, repeatedly attempting to repossess May and sabotage Kazuya, though his schemes typically fail in comedic fashion. He also enlists the help of the cyberdoll Sara by bribing her with ramen. Initially hostile toward May in her small form, his feelings change dramatically when she is enlarged to human size; he falls deeply in love with her and becomes infatuated. This shift in attitude marks a key development, as Nanbara begins to show a softer, more vulnerable side beneath his competitive exterior. Over the course of the series, his relationship with Kazuya evolves from pure rivalry into a more complex dynamic, ultimately revealed to be the foundation of a future partnership. It is later disclosed that Nanbara and Kazuya together found the Cyberdyne Corporation, which goes on to produce all subsequent cyberdolls, and the company is later run by their descendants. His notable abilities include advanced computer programming and an aptitude for creating viruses, along with the financial resources to acquire high-end technology and employ multiple cyberdolls for his schemes.