Rythem is a female J-pop duo that made its name by contributing songs to high-profile anime series rather than relying on their chart performance of their releases. The band comprises high school friends Yui Nitsu and Yukari Kato, both of whom act as vocalists and songwriters in addition to playing piano (Nitsu) and acoustic guitar (Kato). Rythem got its start in 2003 when their tune Hitoritabi Syararuran won the Sony Music audition, bringing them a contract with the label. Their music for the ultra-popular anime series Naruto served as a launch pad for their career; their debut single Harumonia (2003) provided one of numerous theme songs for the series. Their next three singles failed to chart so well, but "Mangekyo Kirakira" (2004) entered the Top 20 thanks to its tie-in with the drama Hikari to Tomo Ni, and their debut album, Utatane (2004), claimed the number six slot on the Oricon charts (it remains Rythem's best achievement). Their next single, Houki Gumo (2005), as well as Kokorobeedama (2006), were both featured in another popular anime series, Yakitate!! Japan, and were later included on the band's 2006 album Mugen Factory, which also sported the track "Negai," which was used in the TV show Sukkiri. The stream of singles stopped for the rest of 2006 as the band hit the road with an acoustic tour, but they returned to the studio in 2007 and released the single Sakura Uta, used in the anime Deltora Quest, and proceeded to record their third album, 23 (the age of both bandmembers at the time), which entered the Oricon Top 20 upon its release in 2008.