[6] After he released his second album, Remember, in 2000, Kim received an offer to record the title track, "Once Upon a Day" in English and to market it in the United States.

[6] The English version of the song, which was named, "Hello Goodbye Hello," reached number 51 on the Billboard Hot Singles Sales chart dated December 22, 2001.

[3][4] Kim's big break in South Korea came in 2003, when the title track from his 2002 album, I Miss You, was included on the soundtrack for the Korean drama, Stairway to Heaven.

[9] From April 2006 to October 2008, Kim served as an "entertainment soldier" under the Ministry of National Defense to complete his mandatory military service.

Kwak Jin-sung of OhmyNews felt that the various artists featured on the album such as Kim Yu-bin and Younha were intended to help attract listeners.

[11] In December of the same year, he released "A Story Sadder Than Sadness (슬픔보다 더 슬픈 이야기)" as the titular song of the drama film More Than Blue.

He collaborated with Girls' Generation member Taeyeon for the single "Different (달라)" in January 2011 before appearing in the first season of the singing competition show I Am a Singer wherein South Korea's most established vocalists competed against each other, months later.

[22] In February 2015, he sang for the SBS romantic comedy television series Hyde Jekyll, Me's official soundtrack and released "Only You (오직 너만)".

[25][26] Kim released a remake of Shin Hyo-beom's 1996 song "I Love You (난 널 사랑해)" for his single album re.MAKE20 #1 on April 26, 2018.

[27] "Kim-Na-Park-Lee" is a known phrase in South Korea which refers to Kim Bum-soo, Naul, Park Hyo-shin and Lee Soo who are considered the country's top vocalists.