Sawai made her acting debut at age 11 in 2004 in a Tokyo stage production of Annie as the titular character, which was simulcast on Nippon TV. Sawai successfully auditioned for the entertainment conglomerate Avex Inc. in 2006, earning her a management contract and training at their in-house music and dance bootcamp. Following this, Sawai continued her acting career with a minor role in the television drama Our Love Song (2007), and made her Hollywood feature film debut in a supporting role in James McTeigue's martial arts film Ninja Assassin (2009), in which she played Kiriko, a rebellious young ninja.
Sawai continued to receive vocal and dancing lessons throughout high school, and later enrolled at Sophia University. While attending university, Sawai was signed by the Avex Trax record label and in March 2012, she performed the United States national anthem at the Tokyo Dome to begin the 2012 Major League Baseball season. A month later, Sawai was announced as one of the members of the girl group ARA, an acronym for Avex Rising Angels, and that November they released the music video for their first and only single "Make My Dreams Come True". The group disbanded in early 2013 after less than a year of promotions, but in July that year, Avex announced that Sawai would be re-debuting as one of the lead vocalists for the girl group Faky under the Rhythm Zone record label. They debuted with the single "Better Without You" on July 29, 2013.
While performing as a member of Faky, Sawai participated in a number of solo projects. In October 2015, she appeared as the lead actress in Elliott Yamin's music video for his song "Katy". Sawai was a featured artist alongside fellow bandmate Akina on Yamato's debut single "Shining", which was released on April 26, 2017. Later that year, Sawai had a voice role as Angélique Noir in the 2017 fashion simulation video game Style Savvy: Styling Star and performed the main theme song for the game "Girls Be Ambitious". In 2018, she made a return to acting with a supporting role in the coming-of-age mystery series Colors, appearing in two episodes. In May 2018, she appeared as a dancer in a limited run stage production of The Book, directed by Fuko Takenaka, as part of the Think Tank Bang dance art collective.
On November 16, 2018, Faky announced Sawai's departure from the group, citing her desire to focus on her acting career. She performed with the group for the last time on December 20 as part of their Four headline tour.
