Tooru Kouno is the main protagonist of Princess Princess D. He transfers to the all-boys Fujimori Academy after the death of his parents in an accident, moving in with his uncle and aunt. A significant reason for his transfer is to distance himself from his younger step-sister Sayaka, who has developed an intense and obsessive romantic attachment to him. Upon enrolling, his androgynous and attractive appearance leads to his recruitment into the school's Princess system, a tradition where the most beautiful first-year students dress as girls to entertain the student body and alleviate tension. Tooru enthusiastically accepts the role, initially drawn by the material luxuries and special treatment that come with the position, and becomes one of the three Princesses alongside the flamboyant Yuujiro Shihoudani and the reluctant Mikoto Yutaka.

Personality wise, Tooru is friendly, sociable, and mischievous. He fully commits to his role as a Princess and uses it as a vital escape from the stress and discomfort of his complicated family life. He often teams up with Yuujiro to tease the more serious Mikoto, delighting in their playful antagonism. Despite his cheerful demeanor at school, he is deeply reluctant to confront his home situation, preferring the safety and distraction of his Princess duties.

His most significant relationship is with Yuujiro Shihoudani. The two share a remarkably close bond that is heavily charged with romantic ambiguity. Yuujiro frequently jokes that they are lovers, and their physical interactions, including a staged kiss intended to scare off Sayaka, reveal a deep and complicated emotional connection that develops over the course of the story. With Mikoto, Tooru maintains a friendly but antagonistic rapport, frequently acting as the instigator of the group's antics. He also becomes friends with the kind and respected Akira Sakamoto.

Throughout the series, Tooru's development centers on his transition from using the Princess program purely as an escapist distraction to genuinely finding a sense of belonging, stability, and support within the bonds he forms at school. His primary ability is his natural talent for performing in his role as a Princess, displaying a commitment to the identity that makes him highly effective and beloved within the academy's unusual social structure, allowing him to thrive in an environment that is starkly different from his troubled home life.