Moon Sua (문수아) is a South Korean rapper, singer, and songwriter under MYSTIC Story. She is currently a member of the girl group Billlie. She is also the younger sister of ASTRO's Moonbin.
Before debuting, Moon Sua is already known by fans as a trainee under YG Entertainment. She had been training at YG Entertainment since 2009 and was speculated to be part a trainee group that is meant to be a younger version of 2NE1. It is also widely believed that she was one of the candidates to become a member of BLACKPINK. After 10 years with the agency, Sua eventually left the agency in 2019 and ended up joining MYSTIC Story.
In 2015, while still a trainee at YG Entertainment, Moon Sua competed in the show Unpretty Rapstar 2 where she finished third place.
After joining MYSTIC Story, Moon Sua continued to train and eventually became a member of the trainee group Mystic Rookies. She eventually made her official debut as a member of Billlie on November 10, 2021 with the mini album The Billage of Perception : Chapter One.