She was born on March 18, 1999 in Seoul, South Korea.
Koeun said in a Ceci 2015 interview that she was a Track and Field athlete in elementary school. She was a registered 80m track and field athlete in the Korea Association of Athletic Federations, who represented Gyeonggi Bongilcheon Elementary School in Paju, Gyeonggi Province. She stopped competing now and is no longer re-registered, so her KAAF profile says she is a "retired player."
Koeun joined SM Entertainment in 2011, training for 7 years as of 2018. She was part of the variety show Mickey Mouse Club broadcasted through Disney Channel Korea in 2015. In 2016, she was part of the "Who's the best?" segment of My SMT along with fellow Rookies Hina and Ningning, and former Rookie Yiyang.
Later in 2017, Koeun with fellow SM Rookies Hina and Ningning sang several OST for the MBC animated series titled Shining Star. They sang the opening theme, "Shining Star", and original songs titled "Dream Dream Dream", "Someday" and "Run and Run." They also covered Girls' Generation's "Into The New World", "Gee", f(x)'s "Ice Cream", BoA's "Moto", and SES' "I'm Your Girl." It was confirmed that one of the songs of Shining Star, Girls' Generation-TTS' "Checkmate" was covered and sung alone by Koeun. Koeun also sang with Onestar for a duet version of "Someday."
"Think positively, act aggressively."
"The cute maknae Koeun is now Ko-leader."