Mirei Masaki, an orphan, was adopted and raised by Yukitaka Jinsei, head of the global Jinsei Corporation, with aid from Yukitaka's associate Daisuke. Her adoption served a precise objective: to operate openly and discreetly as a protector for Yukitaka's daughter, Orihime, heir to the family enterprise. This mission necessitates her enrollment at Saint Ceral Girls' Academy, Orihime's school, where she functions as an undercover bodyguard.

The task poses a formidable challenge since Saint Ceral admits only female students. To infiltrate the academy, Mirei must obscure her biological sex and present a female identity. This transformation becomes a critical juncture in her existence, though its complete ramifications elude her initially.

Physically, she possesses pale skin, stands 161 cm tall, and maintains a slender build fitting her adolescent age. Her most striking traits include vivid orange hair arranged in waist-length twin tails with curtained bangs and a V-shaped fringe, accentuated by green eyes. She routinely wears the school uniform paired with knee-high socks, loafers, and a ribbon hair tie.

Her background focuses exclusively on safeguarding Orihime within the academy's confines.