With a sound influenced by progressive rock and power pop, Eufonius formed in Japan in 2003. Vocalist Riya and keyboardist Hajime Kikuchi met on the internet, began collaborating, and independently released their self-titled debut album, Eufonius, in 2003. "Guruguru," the band's first single, followed in 2004 and reappeared in 2005 on the album Eufonius+. Meanwhile, Eufonius reached the Oricon Singles chart for the first time with "Habataku Mirai," which reached number 80 and marked the band's first release for King Records. Remaining with King Records, Eufonius released albums like 2006's Subarashiki Sekai, 2007's Σ, and 2008's Metro Chrome, with Metro Chrome charting at number 61. 2009's Nejimaki Musica didn't fare as well, stalling at number 161, but the band earned a career-high placement later that year with Ao no Scape, which hit number 55. Eufonius also earned a handful of Top 40 singles on the Oricon Singles chart, including 2007's "Mag Mell / Dango Daikazoku" (number 18), 2008's "Reflectier" (number 24), 2009's "Phosphorus" (number 27), 2010's "Hiyoku no Hane" (number 16), and 2012's "Paradigm" (number 23). After releasing the album Phols in 2021, Eufonius returned to the charts in 2023, when the previously-released single "titillate" returned to the Top 100 Deezer chart in Japan, reaching number 44.