Yoon Sangwoo is a 190cm tall medical student and elite swimmer attending a prestigious "Ivy League" university on a full scholarship. He is introspective, shy, and gentle. He first met Yul-seo at her night job when friends paid for his first experience, but he chose her because he recognized her from the café where he studied. He initially only wanted to talk; Yul-seo was the one who initiated their intimacy. He is the only "boyfriend" who views her with pure concern rather than as an object, frequently visiting the café on slow days just to ensure she isn't being harassed by "rough" men.