Kei Kobayashi (小林桂) is a Tokyo-born jazz vocalist who became a national name in Japan at 20 and kept making records for the next two decades. He was born on May 9, 1979, the second son of a family that worked in jazz on both sides. His mother, Kyoko Murakami (村上京子), is a jazz vocalist, his father, Yo Kobayashi (小林洋), is a jazz pianist and arranger, and his grandfather, Kazunori Murakami (村上一徳), played jazz steel guitar.

His own biography says he began classical ballet at five and took the stage in musicals before he was a teenager, and that he taught himself jazz drums, piano and singing, often singing standards while he played. By 15 he was working Tokyo live houses as a drummer and vocalist. In 1995, still a teenager, he sang at the Kobe charity concert for the victims of the Hanshin-Awaji earthquake, on a bill that included Herbie Hancock. His first CD, a mini-album called Take the "K" Train (K列車で行こう 17歳のジャズ), came out on the Unicom label when he was 17. His first full-length album, 18 / P.S. I Love You, followed on Next Records in 1998 with a small Tokyo band.

The breakthrough came in 1999. At 20 he topped the male jazz vocalist category of Swing Journal's readers' poll, and in December he made his major-label debut on Somethin' Else, a Toshiba EMI imprint, with So Nice. The Japanese press began calling him a "genius jazz vocalist," and over the next three years he released four more albums on the label: Just You (2000), Wonderland (Kei Sings Xmas) and Misty K (both 2001) and Softly (2002). The prizes came quickly. He received the Japan Jazz Vocal Award grand prize in 2000 and Swing Journal's Jazz Disc Award New Star prize in 2001. By his own account he took the magazine's Jazz Disc Award for vocals in 2002 and again in 2003, won the Japan Gold Disc Award for jazz album of the year in 2002, and received an excellence award in the record division of the Agency for Cultural Affairs Arts Festival for Softly.

In 2001 he led his own group at Blue Note New York, which his site describes as the youngest act to do so, and NHK followed the trip for a two-hour documentary titled Kei Kobayashi, the Miracle Jazz Vocalist. That same year he voiced the boy Kenichi in the Osamu Tezuka adaptation Metropolis, written by Katsuhiro Otomo. Later work outside the studio included an appearance as a jazz singer in a 2003 Fuji TV drama, a Shiseido television commercial in 2004 for which he sang "Na-Ga-Shi-Me Play" (written by Yoko Kanno), and a jazz arrangement of the Japanese national anthem sung at baseball events with his father at the piano.

The Somethin' Else period closed with Nature Boy (2004), 19 / My Romance (recorded at 19 and released in 2004) and Love Letters (2005). In 2010 he moved to Pony Canyon and started his own label there, twinKle note. Just Sing (January 2010) and Scenes (November 2010), an album of songs with Japanese titles, came first, followed by a duet-format record with his father, It's a Pity to Say Goodnight (2012), and When I Wish Upon a Star (2013), a Disney collection that marked twenty years of his musical career. He then returned to the repertoire he is best known for with The Standards (2016), gathered from the songs audiences request at his concerts, and The Standards II (2017), his twentieth album. In 2018 he marked 25 years in music with a tour led by the Kei Kobayashi Quintet, and in January 2021 a four-CD box of 74 tracks drawn from his Toshiba EMI albums was released.

On July 10, 2022, he announced on his official site an indefinite break from performing. He explained that he had lived for several years with dystonia of the throat (spasmodic dysphonia) and would use the time for rehabilitation and rest. His last concert before the break was a Nikkei Muse Salon appearance with his band at Nikkei Hall in Tokyo on July 8, 2022. No return has been announced, and his albums remain available.

Kobayashi is a standards singer in the mainstream vocal jazz tradition. Japanese Wikipedia describes his approach as an interpretation that preserves the beauty of the original song, with scat improvisation that puts him on equal terms with the instrumentalists. His records are built from the Great American Songbook, with detours into Disney songs, Christmas songs and Japanese-language film themes, and they are typically made with a working rhythm section and a tenor saxophone: Toru Ambo's horn appears on eight of the albums.

His official biography names the musicians he grew up listening to: Duke Ellington, Oscar Peterson, Bill Evans, Clifford Brown, Stan Getz, Frank Wess, Ed Thigpen, Nat King Cole, Johnny Hartman, Mel Tormé, Ella Fitzgerald, Carmen McRae, Carol Sloane and Maxine Sullivan, alongside the pop of Michael Jackson and the Carpenters.

The family thread runs through the credits. His father, Yo Kobayashi, is the arranger or music director on Softly, Nature Boy, Love Letters, Just Sing, When I Wish Upon a Star and The Standards II, and plays piano on several of them.