Seiya Kori has long silver-blue hair and icy-blue eyes, usually seen with a smile; a flashback showed him once wearing a ponytail. He presents a charming, flamboyant, and prideful demeanor typical of a high school prince, yet reveals underlying kindness and helpfulness. The death of his sister in a car accident instilled deep guilt, as he believes his presence and ability could have prevented it, fueling feelings of inadequacy beneath his confidence.

As a recurring classmate of Nana Hiiragi, he interacts regularly with others. His talent is cryokinesis, allowing him to manipulate and freeze inorganic objects without limit, demonstrated by freezing an entire lake; this power cannot affect living beings.

Following a time skip, his personality drastically shifts. He becomes a gloomy, depressed sculptor, his vibrancy extinguished by meager island rations and an obsessive fixation on his ex-girlfriend Shiori after she abandoned him, highlighting his emotional fragility and need for external validation. His demeanor transforms again when Shiori returns, persuaded by Moguo Iijima to reconcile, restoring his former sparkle and smile. This arc traces his path from confident yet conflicted individual, through diminished despair, and back to his original self via resolved relationships.