It was the first single, "Rakuen" (楽園, Paradise), that established him as a major player in the Japanese sales charts and overseas.
Hirai was voted "Best New Japanese Act" in a pop poll organized by the RTHK radio station in Hong Kong.
In the summer of that same year, Hirai earned the title of Best Male Artist at the inaugural MTV Video Music Awards Japan, and appeared at the official FIFA World Cup 2002 concert with Chemistry and Lauryn Hill.
His 5th album Life Is..., which contains "Strawberry Sex", "Ring", and "Ōki na Furudokei", was released on January 22, 2003.
[5] Hirai's 2005 single, "Pop Star", was one of his career's biggest hits, reaching number one on the Oricon charts.
Hirai then released his 29th single, "Itsuka Hanareru Hi ga Kite mo" (いつか離れる日が来ても, If One Day I'm Separated From You) on April 23, 2008.
On May 27, 2009, Hirai released Ken's Bar II, continuing the concept of creating jazz covers of his favorite Japanese and English songs; these included "Love: Destiny" (Ayumi Hamasaki), "White Lovers" (Keisuke Kawata), and "Because of You" (Ne-Yo).
On September 23, 2009, his 30th single, "Candy" was released, followed by "Boku wa Kimi ni Koi wo Suru" (僕は君に恋をする, "I'm Going to Fall in Love With You") in October.
In 2014, he released the third in his series of song covers, Ken's Bar III, and a collaborative single with Namie Amuro called Grotesque.
In 2021, Anata ni Naritakatta (あなたになりたかった, I Wanted to be You) was announced as his eleventh studio album, released to commemorate his twenty-fifth anniversary.