Few K-pop debut stories encapsulate the industry's brutal volatility quite like that of Lee Young-seo. Before she ever stepped onto a stage as a member of ALLDAY PROJECT, the nineteen-year-old vocalist had already navigated a seven-year journey through four of South Korea's most prominent entertainment agencies—a trajectory marked by near-debuts, unexpected departures, and the kind of quiet perseverance that rarely makes headlines until success finally arrives.1

Youngseo's path into the industry began in childhood, when she was cast in front of her elementary school. By 2018, still in middle school, she had joined SM Entertainment as a trainee—the same agency that would produce groups like aespa. Her time there was brief; she subsequently moved through P NATION in 2019 before landing at Source Music, a HYBE subsidiary then quietly assembling what would become one of K-pop's most disruptive acts.2

At Source Music, Youngseo became part of NTeam—a pre-debut group that trained alongside the trainees who would eventually debut as NewJeans. For nearly four years, from 2019 to 2022, she honed her skills in the same practice rooms and shared dormitories with future members of one of the industry's biggest breakthrough acts. When the project transferred to ADOR under Min Hee-jin's creative direction, Youngseo remained in the trainee pool—until she wasn't. After the lineup was finalized without her, she departed Source Music in 2022, her debut postponed once again.3

Youngseo's fortunes appeared to shift dramatically in June 2023, when she was introduced as a contestant on "R U Next?"—a survival competition produced by JTBC and Belift Lab designed to form a new girl group under the HYBE umbrella. For viewers unfamiliar with her trainee history, she emerged as a revelation: poised vocals, feline visuals that earned her the nickname "Nyangseo" (a portmanteau of the Korean onomatopoeia for a cat's meow and her name), and the kind of stage presence that suggested years of preparation.4

By the September 1, 2023 finale, Youngseo had secured second place overall—selected as one of two members chosen directly by fan votes—and was officially announced as part of the six-member lineup for I'LL-IT (later rebranded as ILLIT). For a trainee who had watched peers debut from three different agencies while she remained in the practice room, the moment represented long-awaited vindication.5

Then came January 5, 2024. Belift Lab released a terse statement: "Following in-depth discussions about future activities, BELIFT LAB and Youngseo have come to a mutual agreement to terminate the exclusive contract. This mutual decision has been made after careful consideration, honoring the artist's wishes. We kindly ask for no unwarranted speculations or misunderstandings." ILLIT would proceed as a five-member group. No further explanation was offered.6

The announcement stunned fans who had followed her journey through the survival show. Speculation swirled across online communities—had she been poached by another agency? Were there creative differences? Health concerns? The silence from both parties only amplified the uncertainty. Youngseo vanished from public view, her Weverse messages having gone quiet weeks before the official announcement, her future in the industry suddenly unclear.7

What happened next defied the typical trainee playbook. Rather than waiting to be recruited or relying on industry connections, Youngseo took an unconventional approach: she sent a direct message to The Black Label's official Instagram account. "I had been with one company for so long, and when I finally left, I was a bit old for an idol," she later explained in the group's pre-debut documentary "DAY 1." "So I was like, 'What should I do?' I had all these concerns. Then I thought, 'I have to step up. I need to go all out for this myself.'"8

The gamble paid off. The Black Label responded, inviting her to a personal audition where she prepared songs and choreography of her choosing. She passed. On June 6, 2025—nearly eighteen months after her departure from Belift Lab—Daily Sports broke the news that Youngseo had signed with Teddy Park's agency and was preparing to debut with an upcoming co-ed group. Three days later, she was officially revealed as a member of ALLDAY PROJECT.9

"Because I went through all those long years, I think that's why I'm able to have such a great result now," she reflected in the documentary. "The first thought I had when I met the team was, 'This is a done deal.' When we come together, we become an incredibly powerful team that just looked so beautiful."10

On June 23, 2025, Youngseo finally made her official debut as the vocalist and maknae of ALLDAY PROJECT—seven years after first entering the trainee system. The group's single album "Famous" entered the market with immediate impact: within four days, the title track claimed the number one position on Melon's Top 100, making it the fastest fifth-generation debut song to achieve this milestone. The track also debuted at number 94 on Billboard's Global 200, marking the first chart entry for a co-ed K-pop group on the ranking.11

Ten days after debut, ALLDAY PROJECT secured their first music show victory on Mnet's M Countdown—a remarkable achievement that validated both the group's commercial appeal and Teddy Park's decision to assemble a mixed-gender act in an industry that had largely abandoned the format. For Youngseo specifically, the win represented closure on years of uncertainty: a trainee who had watched NewJeans explode and ILLIT's "Magnetic" dominate charts was finally standing on stage with a trophy of her own.12

Her reunion with former colleagues at music shows became a recurring topic of discussion among fans. At a June 29 Inkigayo broadcast where ILLIT won for "Do The Dance," cameras captured Youngseo jumping in celebration alongside ILLIT members Yunah and Jeemin backstage—a moment fans seized upon as evidence that no bad blood existed between her and her former group. "I hope y'all stop making up imaginary beef between ILLIT and Youngseo now," one viral post read, accompanying footage of the interaction.13

The year-end awards circuit confirmed what the charts had already suggested: Youngseo's debut year had exceeded expectations. At the 2025 Asia Artist Awards on December 6, ALLDAY PROJECT claimed the New Wave Award and Rookie of the Year. At the 2025 Melon Music Awards on December 20, held at Gocheok Sky Dome in Seoul, the group accepted another Rookie of the Year trophy. Member Woochan delivered the acceptance speech: "I would like to thank our CEO, Teddy hyung, Vince hyung, the visual team, the planning team, and all staff at The Black Label who helped us stand on this stage. Starting with this award, we will work even harder and continue to grow as ALLDAY PROJECT."14

For observers who had followed Youngseo's journey from NTeam to "R U Next?" to ALLDAY PROJECT, the awards represented more than industry recognition—they were proof that persistence, even in an industry notorious for discarding trainees who fail to debut on schedule, could still yield results. At nineteen, she had outlasted the typical trainee timeline, navigated public disappointment, and ultimately found her place in a group that defied industry conventions about what could succeed in modern K-pop.15

"Every time there was news of a new debut group, people would speculate whether Youngseo would be in that lineup," she recalled in "DAY 1," referencing the period after her ILLIT departure. The speculation is over now. Seven years after being cast outside her elementary school, Lee Young-seo is no longer a trainee, a near-debut, or a rumor—she is, finally and definitively, an idol.