Baek released her first single and first self-composed song, "Shouldn't Have" (featuring Day6's Young K), on May 20, 2015. The song went on to become a sleeper hit, reaching number one on several Korean music real-time charts a month after its release among fierce competition. Following the success of the song, she began promotions for it on music programs beginning with Show! Music Core on June 13. Later in the year, she contributed her vocals to the song "So We Are" for the SBS drama Yong-pal.
