Tiffany Young and Choi Jae-rim, each other's prime cell: 'We are all masters of our own universe'

"I am a universe of my own." — from the musical "Yumi's Cells"

At least 389,211 of them. The village of Yumi teems with countless cells, each with its own rank and role. There are the rational and emotional cells that shape her every waking moment; the love cell that turns ordinary days into sweet spring or bitter war; the diet cell reining in her appetite; the pathetic cell that surfaces whenever she fights with her boyfriend Gu-woong; and the sneaky cell that governs even her desires. They share a single purpose: to make Yumi happy. And above them all reigns the prime cell.

Now, one cell has emerged that never appeared in the webtoon — which has racked up 3.5 billion views — nor in the drama series that earned lavish praise for its pitch-perfect casting of actor Kim Go-eun. Its name is simply 109. Called "Baekgu" because it is a trainee cell — and because the Korean word for trainee (gyeonseup) happens to share a character with the word for dog (gyeon) — the character even draws a nod to the EBS television program "There Are No Bad Dogs." The musical, which retells the story from the perspective of Yumi's cells rather than Yumi herself, runs through Aug. 23 at Seoul Arts Center and is unmistakably a fairy tale for grown-ups.

The Korea Herald sat down with Choi Jae-rim and Tiffany Young, who have been each other's prime cell and steadfast support in the rehearsal room and on stage, building a new universe together on a blank canvas. Both said they hoped audiences would use the story's warmth and encouragement to embrace their own inner worlds with a little more kindness.

"Inside my rehearsal folder there are countless files saved as 'May 11 version,' 'June 23 version.' Key changes, lyric rewrites, line edits — they kept coming right up until opening night." — Tiffany Young

It took five years to bring "Yumi's Cells" to the musical stage. Spinning a beloved intellectual property into new content across multiple formats is never easy, and the odds are especially daunting for an original-creation musical premiere. That challenge falls on the creative team and the cast alike.

This is Tiffany's third musical. She had found her footing as a musical theater performer through "Chicago," which she joined around the time the pandemic was easing, but taking on an original creation was a different kind of test. Looking back on the months of work, she said the creative team's belief in her carried her through. "They trusted that the experienced Tiffany could pull it off, and that faith is what kept me moving forward," she said.

The Yumi on stage is a far cry from the glamorous K-pop star life Tiffany has lived. She is an ordinary woman in her 30s, undone by love and buffeted by the small victories and defeats of her working life. "Yumi can't dance well, she's not comfortable in heels — she's a clumsy, everyday person," Tiffany said. "I went back to basics with vocal lessons to build a calm, delicate tone that matched the texture of the character."

Choi's Cell 109 is an original creation that exists only in the musical. He took it upon himself, alongside double-cast partner Jeong Taek-un, to lead the development of the character's ideas. "Director Yang Jeong-ung tends to give the actors a lot of freedom, so sometimes eleven different ideas would pour out for a single scene," Choi said. "Within the physical constraints of time, my job was to gather all of that and help us land on the most effective blocking and lines for the stage."

Cell 109 is everywhere — he never leaves the stage, watching the senior cells of the village as he searches for his own identity. The physical demands are considerable. "After singing the Act 1 closing number '109 Reprise' and leaping into the void, I come offstage drenched — my clothes are completely soaked through," Choi said. "Because I have to wear the same costume again in Act 2, the first thing I do is turn my jacket inside out to hang it and set my hat out to dry. The energy expenditure is significant."

The two actors could not be more different in how they approach a role. Tiffany is a relentless deep-diver; Choi is a meticulous architect.

The first thing Tiffany did after being cast in "Yumi's Cells" was research. "My style is to dig until I hit the bottom," she said. "I explored everything I could get my hands on — I reread the webtoon, went through the merchandise, even read through the comments." She wanted to understand the emotional world of the original fandom. "I fell deep into Yumi's endearing clumsiness, her life at home, the small everyday moments like eating snacks," she said.

For all her thorough analysis, Tiffany says she did not push hard to impose her own vision on the character or the production. "When I'm performing, I want to be an instrument — so I followed the creative team's lead," she said with a laugh, adding that director Yang once told her, "You can speak up if you want to, you know," because she was so quiet. Music, however, was a different matter. As a K-pop artist and singer-songwriter, she contributed actively on the musical side — including playing a decisive role in changing the chord structure of her first solo number, "Ticking."

"Originally 'Ticking' opened in a minor key, and Yumi came across as far too gloomy. But I felt that if Yumi turns dark at that point, it wouldn't fit the finale's narrative arc — so I suggested to the composer that we switch to a major key." — Tiffany Young

To build Cell 109 — named after Yumi's birthday, Jan. 9 — Choi mapped out a precise arc of growth: a carefree, immature trainee cell who gradually discovers the self-worth that is also central to Choi's own identity.

"Self-worth isn't groundless delusion," Choi said. "It's completed when you keep a clear-eyed view of reality and work to fill in what's lacking. In Act 1, 109 is an innocent child who acts on impulse. In Act 2, he transforms into a cell of genuine self-worth — one who persuades others, reflects deeply, and embraces them — and I built the performance step by step to show that transformation."

Where the two actors' interpretations meet, Yumi's breakup scene on stage becomes not a sinking into grief but a self-affirming declaration. "Tiffany's Yumi reclaims her self-worth and delivers a breakup that is confident and resolute — 'I will no longer be dragged along; I will live for myself,'" Choi said.

Cell 109 is, in practice, the true protagonist of "Yumi's Cells." Choi has worked on countless productions — "Chicago," "The Phantom of the Opera," "Les Misérables" — but this role is something different. When the curtain rises on the cell village and he finally appears a few numbers in, the air on stage shifts noticeably. "Jae-rim's Cell 109 is strangely adorable," Tiffany said. "He carries this bright, infectious energy." Once again, he has found the perfect fit.

Behind Choi's celebrated vocals, performance and command of the stage lies relentless hard work. In recent years he has been hailed as the most prolific actor in Korean musical theater and has enjoyed a peak in his career, but overlapping schedules took a toll on his condition. "I wish it hadn't happened, but it did," he said, reflecting on the period. He has been taking vocal lessons again since February.

"In my late 20s and early 30s I relied purely on physical power to produce sound, and I wanted to move past that — to fill in what age was starting to take away. As the years piled up and people around me kept saying 'you're so good, you're so good,' there were also parts of myself I needed to examine more critically." — Choi Jae-rim

Even now, standing at the top of his field, Choi continues to push himself. He even submitted a master's thesis on physical training methods to relieve vocal constriction in musical theater performers.

Tiffany, too, keeps reinventing herself and expanding her range. Her driving force is good content. "From Girls' Generation's debut track 'Into the New World' to TaeTiSeo's 'Twinkle' to the script of 'Yumi's Cells' — the process of seeking out stories that move me and analyzing them has always been my greatest motivation," she said.

"Chasing good stories always leads me to an environment where good people gather," she said, adding that she draws energy from working alongside others to bring a great story to life, even when there are shortcomings along the way. Musical theater performers Choi Jeong-won, Jeong Yeong-ju and Ivy are among those people — as is Choi Jae-rim, her partner once again after "Chicago."

"Jae-rim oppa is my real-life Cell 109," Tiffany said. "His vocal choices and the way he has built his career are a huge inspiration to me. Every time the music changed, I would ask him to sing my numbers for me so I could listen to the recordings and work on my vocal tone."

Choi, for his part, has watched Tiffany grow as a musical theater performer more closely than most. "Tiffany is an actor who brings the showmanship she built leading a global career from a young age, along with real entrepreneurial insight," he said. "As a colleague, I'm constantly struck by her ability to design her character — Yumi — with such clean precision, and by the sheer tenacity with which she invests her time."

"Yumi's Cells" has expanded from webtoon to drama series to the stage, becoming a defining example of single-source, multi-platform storytelling. According to producer Sam Company, representatives from Toho — Japan's largest entertainment company — and NHK attended performances and were effusive in their praise. "Yumi's Cells carries a universal emotional truth that anyone, East or West, can connect with," Choi said. "It has everything it needs to travel abroad."

"In the thick of daily life, there are many moments when you feel alone. But inside each of us live countless cells straining and struggling on our behalf. When you realize that you may feel alone but are never truly alone — that is when deep comfort arrives." — Choi Jae-rim