Ki Hui-hyeon (born June 16, 1995), better known by the mononym Huihyun and former stage name Cathy, is a South Korean singer and rapper. She is best known as a member of the South Korean girl group DIA.
She is a former Woollim Entertainment trainee and trained with members of the girl group Lovelyz.
In February 2015, MBK Entertainment announced its plans to debut a new girl group under survival program through which the public could become familiar with the group members. In June, MBK announced that they had canceled their plans for a survival program, and decided to select the members internally. MBK Entertainment revealed the final line-up and the group name.
Ki debuted under stage name Cathy as the main rapper of DIA. The group made their official debut stage on the music program Mnet’s M Countdown on September 17, 2015. Shortly after debut with the group in December 2015, MBK Entertainment announced she had temporarily withdrawn from the group, along with her bandmate Jung Chae-yeon, to represent MBK Entertainment on reality girl group survival show Produce 101 for the chance to debut in a Mnet girl group. She auditioned and signed contracts with the show before DIA’s official debut. She is the current leader of the group, starting from the promotions of the mini album Spell.
In 2016, she also appeared on Mnet’s rap survival program Unpretty Rapstar 3 as a contestant.