Ouji Kōsaka, a classmate of Yukiteru Amano, begins as a teasing antagonist whose light bullying evolves into cautious alliance during the Survival Game. His self-aggrandizing "Kosaka King Diary," documenting fabricated triumphs, transforms into the "Neo King Kosaka Diary" after introspection, now acknowledging allies’ contributions—a testament to his growing embrace of teamwork. Early in the game, he betrays Yukiteru by surrendering him to terrorist Minene Uryu during her school assault, exposing his opportunistic instincts. Yet interactions with Hinata Hino, Mao Nonosaka, and Aru Akise gradually unveil his collaborative side: he aids crime-scene investigations and devises survival tactics, such as barricading windows against the Tenth Diary Holder’s feral hounds. Skeptical of Yukiteru’s capabilities, he shifts toward reluctant cooperation upon witnessing the Future Diaries’ power.

A defining shift occurs when he shields Yukiteru and Hinata from Yuno Gasai, prioritizing their safety over self-interest. This act crystallizes his internal conflict between pride and responsibility, mirrored in his diary’s progression from solipsism to collective recognition. Despite joining group efforts with pragmatic resolve, his dynamic with Yukiteru remains contentious; he openly critiques Yukiteru’s dependence on Yuno and perceived indecisiveness, challenging his leadership and moral choices.

The irony of his name "Ouji" ("Prince") surfaces in *Future Diary Redial* when characters mistakenly assume Tsubaki Kasugano’s mention of a "prince" refers to him—a playful nod to his self-styled "king" persona, juxtaposing his grandiose self-image with his flawed, evolving humanity.