The younger Daidou sister, a bossy, serious, and short-tempered girl studying to become a dental hygienist. She fell in love with her elder sister, Ayumu, when they were teenagers, but they grew apart after Ayumu devoted herself to her studies. Jealous and lonely, Sayo began to shun her sister, who left for her classes, leaving her sister and first love behind. Years later, they reunite thanks to a dating website.