Kim's breakout role was in romance melodrama series Piano, in which she played a pure, fragile young woman. The series was the second most successful drama of 2001, achieving a peak rating of 40.2%. She then starred in 2002 hit Romance alongside Kim Jae-won, which led her to stardom and won her Top Excellence Award in acting. In 2003, she raised her profile through the huge box-office hit My Tutor Friend, where she acted as a college girl tasked with helping a delinquent student her own age graduate from high school alongside her opposite Kwon Sang-woo. In 2004, she starred in Too Beautiful to Lie, as an ex-convict who pretends to be the fiancee of a man being pressured to marry by his nosy, close-knit family. Due to the success of her films, Kim was dubbed "the queen of romantic comedies" by the Korean press.