Son Junho Called IU His Future Daughter-in-Law on SNS

On Sunday, IU was doing what many K-drama fans do on a quiet weekend: watching television and reacting to it on social media. What she shared, however, set off a chain of celebrity interactions that quickly became the most talked-about moment of the day — not because of the drama itself, but because of what happened next.

The star of MBC's hit Friday-Saturday drama 21st Century Princess posted on her personal SNS after watching KBS1's Open Music Concert, where celebrated musical theater couple Kim So-hyeon and Son Junho took the stage at KBS Hall in Yeouido. Their performance of "The Phantom of the Opera" was, by all accounts, spectacular. IU called it exactly what fans watching at home were thinking — except she used a slightly unconventional set of names.

"Wow, Queen Uihyeon and King Huijong," IU wrote, tagging a screenshot of the couple mid-performance. For anyone watching 21st Century Princess, the inside joke landed perfectly: Kim So-hyeon and Son Junho both appear in the drama in flashback sequences, playing a royal couple in the show's fictional constitutional monarchy of Korea.

Son Junho's Response Left the Internet Speechless

What turned a charming fan moment into an internet event was Son Junho's reply. The veteran musical actor reposted IU's message with a comment that showed exactly the kind of quick wit that comes from decades on stage: "Ah, my son's girlfriend saw us."

He then added, with escalating mock formality: "Hehe... my son's girlfriend... my daughter-in-law... new daughter-in-law??? What am I supposed to call her when we meet in real life?"

The logic tracks, if you know the show. In 21st Century Princess, IU plays Seong Hee-joo — a commoner turned chaebol heiress who enters a contract marriage with Prince Lee An-dae-gun (played by Byeon Woo-seok). Son Junho's character is King Huijong, the ruling monarch and, narratively, the father-in-law of IU's character. Hence: his son's girlfriend. Hence: the internet's collective meltdown.

Kim So-hyeon and Son Junho are, of course, married in real life — making this a moment where fictional royalty, a real-life couple, and one of Korea's most beloved entertainers collided in the most delightful way possible.

About '21st Century Princess'

21st Century Princess (원제: 21세기 대군부인) is one of MBC's most-watched dramas of the year. Set in an alternate version of South Korea that operates as a constitutional monarchy — where real political power sits with elected officials but a royal family remains at the center of public life — the show imagines what it might look like if a sharp, independent young woman unexpectedly found herself bound to a prince.

IU plays Seong Hee-joo, a woman with a complicated background who enters a contract marriage with Prince Lee An-dae-gun after a series of dramatic circumstances. Byeon Woo-seok, best known internationally for his role in Lovely Runner (2024), brings the prince to life with the kind of quiet intensity that has already earned him a devoted following. The two have undeniable chemistry on screen, and the drama's writing — by Yu Ji-won — has kept viewers returning week after week.

Kim So-hyeon appears in flashback scenes as Queen Uihyeon, the prince's mother. Son Junho plays King Huijong, the monarch whose son Lee An-dae-gun inherited both a royal title and considerable emotional complexity. Their guest appearances, though brief, have been memorable — and IU clearly noticed.

Climbing to 11.2 Percent

The timing of Sunday's SNS moment came against a backdrop of genuine ratings momentum. The most recent episode — episode 8, which aired the previous day on Saturday — pushed 21st Century Princess to its highest viewership yet: 11.2% nationwide, and 11.6% in the Seoul metropolitan area, according to Nielsen Korea. The drama launched at 7.8%, meaning it has climbed steadily across eight episodes.

More striking was a single peak moment: when Lee An-dae-gun shielded Seong Hee-joo from the flash of media cameras during a chaotic public event, the per-minute viewership hit 14.6%. That number placed the scene at number one across all Saturday programming in South Korea — a meaningful benchmark for a drama that only debuted several weeks earlier.

Episode 8's central plot twist — the public exposure of the couple's contract marriage — sent shockwaves through the show's audience. The contrast between the warmth viewers had been watching develop over seven episodes and the sudden, very public crisis gave the episode a particularly gripping quality. Seong Hee-joo, left alone on a stage as her secret became public knowledge, while Lee An-dae-gun fought his way through the chaos to reach her — it was the kind of moment that generates screenshots, rewatches, and Sunday morning discussions about what it all means for the rest of the season.

What the Viral Moment Says About the Drama's Pull

The IU-Son Junho exchange was funny, warm, and completely unscripted. But it was also a reflection of something the show has achieved that few dramas do: it has made its fictional world feel inhabited and alive, even beyond the screen.

When IU — whose character is the drama — instinctively referred to Kim So-hyeon and Son Junho by their character names while watching them perform, it suggested that the story has taken hold in the way that only the best dramas do. The lines between fiction and reality blurred, briefly, in the most charming possible way.

Son Junho's reply took that a step further: not correcting the frame, but leaning into it with self-aware humor. The result was a moment that felt less like a PR opportunity and more like people who genuinely enjoy each other's company playing along with a shared joke.

What Comes Next

21st Century Princess episode 9 airs on May 8 at 9:50 PM KST on MBC. With the contract marriage now exposed and the central couple facing their most public crisis yet, viewers are not expecting the drama to slow down anytime soon.

As for IU and Son Junho's eventual real-life meeting, the actor has already helpfully pre-planned the awkwardness. "What am I supposed to call her?" he wrote, half to the internet, half to himself. Given the way their fans responded — with thousands of comments, reposts, and screenshots — the answer, apparently, is: anything she wants.