Mizuho Yuuki is a 17-year-old female high school student with brown hair and striking red eyes, standing 1.58 meters tall with blood type O and physical measurements of 82-59-87. Born on June 12, she lives with her mother and younger brother, Tatsuya Yuuki. Their home also hosts Tohru Nagase, a classmate boarding with them as gratitude for saving Tatsuya from drowning.

Strong-willed and active, Mizuho possesses a stubborn streak reportedly inherited from her father. She values honesty intensely and refuses to tolerate deception or betrayal. Despite this stubbornness, she remains consistently friendly and fiercely loyal, especially toward her best friend, Miko Kamiyama. Mizuho actively participates in her school's tennis club and enjoys the sport.

Her story revolves around navigating complex romantic feelings while steadfastly prioritizing her friendship with Miko. When Miko confesses her love for Tohru Nagase and asks Mizuho for help expressing it, Mizuho agrees. This decision comes despite Mizuho's own developing feelings for Tohru, feelings she consciously suppresses to avoid betraying Miko. She even accepts dates with Haruhiko Satake, a wealthy classmate harboring strong feelings for her, further complicating matters alongside underclassman Ai Minamizato, who also pursues Tohru.

Living under the same roof forces Mizuho into frequent proximity with Tohru, allowing her to gradually understand and ultimately fall in love with him. She struggles deeply to accept these feelings due to her unwavering commitment to Miko's happiness. Mizuho's persistent efforts to resolve this internal conflict form a central aspect of her narrative across the original OVA and its television series retelling.