The next hit after “I Like It” was “Motherland”, which appeared in the spring of 2004. At that moment, the singer was already a huge celebrity in her native Japan. Some critics said that the success of “Motherland” was accidental since it was the soundtrack for a popular anime series. However, it was impossible to deny the popularity of Kay, which was steadily rising.
Kay’s most successful single to date is “Koi ni Ochitara" (If I Fall In Love), which she recorded in 2005. It appeared in the movie Koi ni Ochitara: Boku no Seiko no Himitsu and earned several platinum certifications.
In 2006, the singer presented a new hit single "Kirakuni/Together", which sold 14,000 copies in Japan. The next single “Shadows of Desire” was recorded in English.
In the spring of 2007, Kay joined m-flo to record a new single “Love Don’t Cry” for the duo’s fifth studio album. The same year, the singer presented her seventh album, All Yours, which earned gold certification.
In the fall of 2007, Crystal presented an EP, Shining, which featured Christmas songs. After the release, Kay slowed down a little, diverting her attention to filming commercials and attending TV shows.
In 2008, Crystal released a new single “One”, which became a soundtrack for the 11th Pokemon movie. In August 2009, Crystal presented her 24th single, "After Love (First Boyfriend) / Girlfriend", which became very successful.
In September 2009, the artist released a compilation album, Best of Crystal Kay, which featured 30 tracks. The limited edition of the album came out with four new songs. A year later, Kay presented a new LP, Spin the Music.
In 2011, Crystal was planning to go on a tour to support the latest album. However, it was canceled due to her health problems.
In October, Kay left Epic and signed a contract with a new label, Delicious Deli Records.
In 2012, the singer presented a new LP, Vivid, which brought her commercial success. The next LP, Shine, came out three years later. The latest album to date, For You, came out in 2018 and reached the 30th position of the Oricon chart.