Se-Jun's twin sister who had heart transplant when she was young due to a weak heart. She was obsessed with her brother Se-Jun and he fell in love with her as well but when he rejected her she warms up to Gi-Hun and becomes a more out-going person.

Spoiler

She sleeps with her brother when he reveals Gi-Hun's secret, but it is shown later that Se-Wa had feelings for Gi-Hun