Umi Hiiragi, the youngest of six sisters, possesses light blue hair frequently styled in long straight lengths, spiky bangs, braids, or antenna-like elements. Her teal tareme eyes create a soft-eyed impression. She stands 165 cm tall with a slim, pale teenage physique and measurements of 86-57-89 cm, blood type AB, and a birthday on May 5.

She exhibits a deredere personality, openly expressing warmth and affection, particularly toward Kuuya, the family's adopted brother. Her constant desire to hug and cuddle him reveals a possessive streak, balanced by diligent and intelligent traits. As a high school student, she engages in computing but displays disastrous cooking skills, transforming even simple dishes into hazardous inedible results.

Within the Hiiragi household, she serves as Kuuya's non-blood-related younger sister, actively providing care and unwavering devotion. Her wardrobe encompasses school uniforms, ribbons, sport bloomers, sweaters, and diverse attire ranging from casual wear to formal kimonos and wedding dresses.