 In 2014, Ku released her 5th digital single "Happy", which is also a remake to Seo In-guk's song "Were We Happy".She released another self-written single titled "Must" the same year, which also serves as the soundtrack of her film "My Daughtr".In 2015, she released a second album titled "Breath 2", which serves as a follow up to her 2009 album "Breath".