Mirei Shizuoka attends Prefectural Umineko Commercial High School as a second-year student and competes on the swim team. She specializes in backstroke and individual medley events, coming close to victory in the 200-meter category at the Kantō Tournament the year before the series begins. Her swimming skill draws admiration from peers, especially male students, though their attention causes her significant unease due to intense self-consciousness about her body, particularly her breast size. This sensitivity results in visible embarrassment, blushing, and avoidance of situations where she might be seen, like changing with teammates or bathing.

Despite a reserved and polite public demeanor, Mirei possesses a contrasting private side marked by repressed erotic inclinations. She habitually wears revealing lingerie, favoring tangas, under her clothes and keeps a collection of phallic-shaped objects such as kokeshi dolls in her bedroom. She experiences sexual fantasies and tends to interpret mundane objects or situations in a sexualized way, fixating on statues or horns reminiscent of male anatomy. This extends to actively seeking chances to observe males undressing, though she remains highly secretive about these tendencies. The origins of this paradox are suggested to lie in her family environment, where her parents openly engage in sexual roleplay, witnessed by Kaname Okiura during a home visit.

Her family background includes considerable wealth and status; her grandfather founded the prominent Shizu Group conglomerate. This privileged upbringing stands in contrast to her timid public personality. While her romantic feelings remain ambiguous throughout the series, she displays occasional possessiveness or interest in Kaname Okiura, never conclusively resolved.

No significant development of these core traits occurs across the manga or anime. Her role primarily provides comedic conflict through the juxtaposition of her innocent public image and her private, sexually charged behavior.