Yui was born in Fukuoka, Japan. She grew up in a single parent family and does not have any memory of her father. She states that she has always been close to music; be it from her mother's car stereo or the radio, she would remember the tunes and sing along. Her professional career began in March 2004 when, at the recommendation of her juku instructors, she applied for an audition hosted by Sony Music Japan. Despite the audition rule that a participant could only perform two songs, Yui was allowed to sing three. She first sang "Why Me" (a song later included in her debut single), followed by "It's Happy Line" and "I Know". The judges gave her the maximum score, causing a fierce scramble among record labels to sign her. Because "I Know" was incomplete at the time, the judges were able to get a glimpse of what would later be dubbed "Yui-go", or Yui-speak (Yui語 in Japanese), nonsensical English hummed to a tune during her songwriting process — an example of Yui-go can be found in the film Midnight Sun when Yui is working on the song "Goodbye Days". On December 24, 2004, she released her debut single "It's Happy Line" under indie label Leaflet Records, coupled with the track "I Know". The pressing was limited to only 2,000 copies in her home area.