Reol began as a musical unit that consisted of vocalist れをる (Reol), musical arranger Giga and movie director/producer Okiku. Prior to the group's official formation, both singer Reol and arranger Giga had worked consistently together, releasing an album, No Title-. The group signed to Toy's Factory in 2015, using the name REOL (stylized RΞOL) for the project, and released their debut studio album as a group, Σ, on October 19, 2016. On August 1st, 2017, REOL announced that they would split, calling it a "developmental breakup". On October 11th, they released their last work as a group, Endless EP. REOL performed together as a group for the last time and disbanded on October 22nd, 2017. On January 9th, 2018, having teased an announcement four days prior, singer Reol announced that she would continue under the name Reol as a solo singer-songwriter signed to the CONNECTONE label, also announcing a mini-album entitled 虚構集 (Kyokōshū) for release in March 2018. The same day, she publicly released a track from the album, 平面鏡 (Heimenkyō), that she said would indicate Reol's future direction. Heimenkyō was arranged by Giga. Reol announced she had recorded a song for HAL Technical School's commercial entitled サイサキ (Saisaki) on March 30th, and released the full track on various platforms on May 16th. Saisaki was again arranged by Giga. On October 15, 2018, Reol released a new major album as her solo act, 事実上 (Jijitsujō). Giga arranged many of the tracks on this album.