MOON SUA X SIYOON (문수아x시윤) are the first sub-unit of the South Korean girl group Billlie (빌리), formed by MYSTIC Story. The duo consists of members Moon Sua and Kim Siyoon.

They debuted on April 7th, 2025, with their first single titled SNAP (feat. sokodomo), featuring two tracks, including the self-titled title track SNAP (feat. sokodomo) and an English version of the song, with both including a participation from the South Korean rapper sokodomo. Read more on Last.fm. User-contributed text is available under the Creative Commons By-SA License; additional terms may apply.