Miyabi Kannagi is one of the main heroines of the series and a classmate of the protagonist, Shougo Mikadono. She has short orange hair and orange eyes, and her family runs a clothing line. She is a member of the swimming club at Miryuin Private Academy. Her background includes a childhood meeting with Shougo while he was recovering from an injury in the hospital; the two played together and even held a pretend wedding, which left a lasting impression on her. In the course of the story, it is revealed through a DNA test that she is not Shougo’s sister but rather his cousin, and in the light novel continuity she later becomes his step-sister after his mother marries her father.

Miyabi’s personality is often described as tsundere. She has a childish appearance that matches her childish, energetic, and moody temperament. In everyday situations she tends to be quiet and reserved, but during important events she becomes noticeably excited and outgoing. Around Shougo, these traits become amplified: she enters what is referred to as an “imouto mode,” acting as a very clingy and possessive little sister. Her motivations are centered on her deep affection for Shougo, and she openly states her desire to marry him, making her one of the primary candidates for his future partner.

Her role in the story is that of a central heroine and a key figure in the mystery surrounding the identity of Shougo’s hidden sister. She is one of the first girls to show a clear romantic interest in him, and her interactions often swing between shy affection and determined pursuit. Over the course of the series, she develops from a somewhat distant classmate into a more openly caring and emotionally vulnerable character, especially as her past connection with Shougo comes to light. Notable abilities include her skill in swimming, her family’s wealth and business influence, and her talent for shifting into a sisterly persona when it suits her goals.