Yeon Woo is a delicate and timid boy, whose feminine features often result in people misgendering him (which is his biggest insecurity).  He has had a one-sided crush on the school's idol, Kang Haesol, for ages yet has never mustered up the courage to even speak with her.  I imagine his voice to be soft and slightly androgynous, as he is often mistaken for a girl even after speaking.