Kim Se-jeong was born on August 28, 1996, in Gimje, North Jeolla Province, but moved to Anyang, Gyeonggi where she stayed with her mother and older brother at her aunt's house. Her parents split when she was a child, leaving her mother to single-handedly raise her and her brother. She has previously stated that she did not contact her father until she was in her third year of middle school.