"Butter-fly" was released on April 23, 1999 under the Index Music label (now T.Y.Entertainment) and is Kōji Wada's first major single release. In additions to the title song, the same single release also includes the song "Seven".
Later on, The "Strong Version" cover and the "tri. Version" of the song, also performed by Wada, were released on April 22, 2009 and November 25, 2015 respectively, with the tri. Version cover being used as the opening theme song for the 6-part anime movie series Digimon Adventure tri. Sadly, Wada passed away on April 2, 2016, five days after the release of his last single "Seven ~tri. Version~".