Kyōya Onodera has messy blue hair and azure eyes. He initially wears a loose-collared navy blue school uniform, later adopting a navy blue watchman’s uniform after a significant timeskip; his appearance remains unchanged due to his inability to age.

Initially aloof and intimidating, Kyōya is socially awkward yet genuinely desires friendship despite struggling to form connections. He uses outdated terms like "omae-san," shows limited knowledge of modern slang, and prefers retro video games and idol music. His logical, analytical nature enables exceptional deductive reasoning, uncovering hidden details like classmates' relationships or suspicious activities. He maintains meticulous journals to compensate for his talent’s drawback.

Kyōya was born to a Talented mother who died mysteriously and a father who perished in a car accident. Relatives refused custody, forcing him and his younger sister Rin to live independently in poverty. He worked multiple jobs to support Rin until she ran away after secretly editing his journals to erase all references to herself, ensuring he would forget her due to his memory limitations. His subsequent transfer to the island school aimed to find Rin, who had disappeared there years earlier.

His primary ability is immortality, granting rapid regeneration from fatal injuries like explosions, poison, or trauma—though he experiences full pain. A major drawback prevents him from retaining memories beyond approximately two years, with minor details fading in as little as two months. To manage this, he documents his life in detailed journals but fears tampering could manipulate his identity. He also lacks a sense of smell, relying on visual labels for food expiration dates, and possesses medical expertise, carrying a first-aid kit and performing autopsies.

Kyōya’s relationship with Nana Hiiragi involves mutual suspicion and intellectual clashes; he consistently doubts her role in student disappearances, engaging in psychological battles to expose her. Despite this, he occasionally extends tentative friendship offers, reflecting his underlying isolation. His bond with Michiru Inukai reveals protective instincts; he entrusts her with his journals, making her the sole person initially aware of his memory limitation. His search for Rin drives him, culminating in an alliance with the shapeshifter Jin—strongly implied to be Rin—who aids his investigations into the island’s conspiracies.

Kyōya evolves from a solitary investigator to a strategic ally. After Nana confesses her role in the murders and the government’s anti-Talented agenda, he prioritizes collective survival over personal grievances, collaborating with her to evacuate and protect remaining students. Post-timeskip, he joins forces with Jin to combat military oppression, committing to end violence against the Talented. His investigative focus shifts from Nana’s crimes to exposing systemic corruption.