Ayase was born on April 4, 1994, in Ube, Yamaguchi Prefecture, Japan. At the age around 3 or 4, he received his first piano lessons by his grandmother who was a music teacher. When he started elementary school, he received professional piano lessons at a music school. In his later grade school years, he also learned how to play acoustic guitar after he was given an acoustic guitar as a Christmas present. The first song he learned to play on guitar was originally written by Ringo Sheena. His early musical influences were Exile, Sukima Switch, Kobukuro, RADWIMPS, and Aiko, J-Pop titans of the time. 

In 2012, when he was 16, Ayase formed his rock band named Davinci with classmates from high school, performing under the name Keiichirō, and being in charge of vocalist and leader of the band. Due to his activity with the group, He later dropped out of high school, and, in 2016, the band relocated to Tokyo. In October 2018, the band went hiatus due to Ayase's medical treatment of peptic ulcer disease. The band announced the disbandment two years later in July 2020. 

in mid 2019, Ayase received an offer from Sony Music Entertainment Japan to collaborate on a project to produce songs inspired by short stories published on creative writing social media Monogatary.com. , which would later form the iconic YOASOBI, where he would continue to develop his Vocaloid and songwriting skills.

Lilas Ikuta was born on September 25, 2000, in Tokyo, Japan, and has three older siblings.  She moved to Chicago, United States in 2001, living there until she was three years old. Ikuta learned piano when she attended the first year of elementary school, and then acoustic guitar in sixth grade. She was also a member of the school's music club from third to sixth grade. Ikuta wrote her first song when she was still at elementary school and performed her first concert when she was a middle school student. She attended College of Art, Nihon University in April 2019, and graduated in March 2023. Ikuta has cited Taylor Swift as her major music influence. 

Ikuta went on participating in several castings and auditions, a notable one being the 2015 "Singin' Japan"  hosted by Sony Music Entertainment Japan, where she was selected as a finalist. She also participated in the new artist training course called the Lesson sponsored by SMEJ and released her demo CD 15 no Omoi. She later became a member of cover group Plusonica until 2021.

From 2021 to 2023, she worked on more singles as a solo singer, a few being  "Answer" for Tokio Marine & Nichido Life Insurance's Anshin Disability Insurance advertisement, "Romance no Yakusoku", originally from her EP Jukebox, on August 14, accompanied love reality series Kyō, Suki ni Narimashita: Himawari-hen, and so on.