Kawai Ruka was born on March 20, 2002, in Yamaguchi Prefecture, Japan, though she spent much of her childhood in Tokyo.1 Her journey into entertainment began remarkably early—at just four years old, she started training in dance, specializing in hip-hop styles that would later become her signature.2

On December 25, 2016, Ruka was introduced as one of the founding members of Shibu3 Project, a Japanese idol group that made its live debut on March 26, 2017.3 As a member of the Blue Class unit, Ruka was specifically in charge of hip-hop dance, showcasing the technical foundation she had built since childhood.2 However, her time with Shibu3 Project would be brief—she departed the group sometime after April 2018 to pursue opportunities in South Korea.3

In 2018, Ruka passed YG Entertainment's Japanese auditions and joined the company as a trainee—a decision that would make history.1 Over the next six years, she became YG's longest-trained BABYMONSTER member, alongside fellow trainees Rora and Asa who also trained for six years.4 During this extensive training period, Ruka worked meticulously on mastering Korean pronunciation for rapping—a challenge for Japanese native speakers that she approached with particular dedication.5

YG producer BIGTONE observed that Ruka possessed an innate "hip-hop feel" that made people perceive her as a rapper even when she wasn't performing. He praised her natural relaxing motions and noted that her Korean pronunciation improved significantly over her trainee years.5 Similarly, renowned choreographer LEEJUNG, who worked with Ruka during training, praised her dance experience and understanding to such a degree that Yang Hyun-suk, YG's founder, reportedly never felt the need to advise her on dancing.6

On December 30, 2022, YG Entertainment unveiled a teaser poster announcing their plans to debut a new girl group.3 The following day, a YouTube video titled "YG NEXT MOVEMENT" confirmed the group's name as BABYMONSTER, with Ruka visible practicing in the footage.3 Between January 12 and February 6, 2023, YG gradually revealed the seven potential members through live performance videos. Ruka was the final member introduced—on February 6, 2023—creating anticipation as fans waited to see who would complete the lineup.3

Her introduction carried historical significance: alongside fellow Japanese member Asa, Ruka became the first Japanese female idol to debut under YG Entertainment.7 While YG had previously debuted Japanese male idols in TREASURE (Haruto, Asahi, and former member Mashiho), Ruka and Asa represented a pioneering moment for Japanese women in the company's roster.1 At age 21, Ruka also held another distinction: she was the oldest member and the only adult when BABYMONSTER debuted, with a remarkable seven-year age gap separating her from the youngest member, Chiquita.1

During the "Last Evaluation" survival series that chronicled BABYMONSTER's formation, Ruka delivered a standout solo performance on April 15, 2023. She covered Masked Wolf's "Astronaut In The Ocean" and 2NE1's "MTBD (Mental Breakdown)" in a powerful medley that showcased her rap skills and stage presence.8 The clean version of her performance, uploaded to BABYMONSTER's official YouTube channel, quickly went viral among international K-pop fans.9

The performance highlighted Ruka's ability to seamlessly blend Western hip-hop with YG's signature rap style. Her mid-to-low tone delivery and flowing rap technique became defining characteristics that distinguished her from fellow main rapper Asa, who handles high-tone rapid-fire sections.10 Together, the two Japanese members formed what many considered one of K-pop's strongest main rapper duos, despite both rapping primarily in Korean as non-native speakers.10

BABYMONSTER debuted as a six-member group on November 27, 2023, with the digital single "Batter Up," as member Ahyeon took a health hiatus.11 The debut made immediate impact: the music video amassed 22.59 million views within 24 hours and became the fastest debut music video to reach 100 million views on YouTube, achieving the milestone in just 18 days.4

On April 1, 2024, BABYMONSTER officially debuted as a complete seven-member group with their first mini-album "BABYMONS7ER," following Ahyeon's return.11 In an interview with The Korea Herald around the album's release, Ruka articulated the group's concept: "We play around like kids when we are not working but we change into monsters when in work mode. We are a group with monstrous charisma."11 As a Japanese member, she expressed particular excitement about the group's upcoming fan meeting tour in Japan, noting it was "an honor to present our performance in Japan and other countries."11

The EP "BABYMONS7ER" received over 460,000 pre-orders and debuted at number three on the Circle Album Chart.4 The title track "SHEESH" peaked at number ten on the Circle Digital Chart and became the highest-charting K-pop girl group entry on Spotify that year.4 Billboard recognized BABYMONSTER's impact, featuring them as "Rookie of the Month" in July 2024, where Ruka shared that member Chan-hyuk from AKMU advised them to "make every stage your own."12

On November 1, 2024, BABYMONSTER released their first full-length studio album "DRIP."13 Among the album's tracks, "Woke Up In Tokyo" held special significance as a duet between the group's two Japanese members, Ruka and Asa, who co-wrote the song's lyrics together.13 The playful track incorporated Japanese phrases, anime references (including Dragon Ball's "Kamehameha"), and cultural touchpoints like yakiniku and Yamaguchi—Ruka's home prefecture.14

"Woke Up In Tokyo" showcased Ruka's growth from performer to creative contributor. The song's lyrics demonstrated her ability to blend her Japanese identity with BABYMONSTER's global hip-hop sound, featuring verses where she introduces herself as "Ruka Kawai desu" and references dreaming while looking for a fun time in Tokyo.14 The album "DRIP" marked new career highs for the group, with 820,000 orders within a week and their first Billboard 200 entry at number 149.4