If you’ve spent any time in the rabbit hole of historical K-Dramas, you’ve probably stumbled across the 2017 MBC hit The Emperor: Owner of the Mask. Most fans just call it Ruler, though. It was one of those shows that had everyone screaming at their TV screens every Wednesday and Thursday night. But looking back, it wasn’t just the plot about a secret society controlling the water supply that kept people hooked. It was the cast of Ruler Master of the Mask. Seriously. This lineup was basically a "who’s who" of future Hallyu royalty.
Honestly, rewatching it now feels like looking at a time capsule of stars right before they went supernova. You have former child actors trying to prove they can lead a "grown-up" drama, idols fighting for respect, and veteran villains who make your skin crawl.
Yoo Seung-ho: The King who wore the mask
Yoo Seung-ho played Crown Prince Lee Sun. He’s the heart of the show. If you don't know Yoo Seung-ho, he was famously dubbed the "Nation's Little Brother" in Korea because he's been acting since he was a literal child in the movie The Way Home. By the time he joined the cast of Ruler Master of the Mask, he had already finished his mandatory military service—something he did earlier than most stars to get it out of the way.
He plays a prince forced to wear a mask from birth. Imagine the acting challenge there. You can’t use half your face! He had to convey grief, regal authority, and romantic yearning entirely through his eyes and his voice. His performance is gritty. He isn't just a pretty face in a hanbok; he portrays a man broken by the realization that his father’s throne was bought with the blood of the people. It’s a heavy role. He carries the weight of the Joseon dynasty on his shoulders, and you can see it in the way he slumps when the mask finally comes off.
Kim So-hyun: More than just a love interest
Then we have Kim So-hyun. She plays Han Ga-eun. Like Yoo Seung-ho, she grew up on screen. You might remember her as the younger version of every lead actress in the early 2010s. In Ruler, she’s the emotional anchor. Some critics at the time felt her character was a bit too "damsel in distress" toward the middle of the series, but Kim So-hyun’s actual performance is rock solid. She brings a certain intelligence to Ga-eun.
She isn't just there to be the Prince's crush. She wants revenge for her father. She wants justice. The chemistry between her and Yoo Seung-ho was a huge talking point during the show’s run. They had worked together briefly years before in I Miss You, so fans were basically manifest destiny-ing this pairing for years. It paid off.
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Kim Myung-soo (L): The peasant who stole the show
Let’s talk about the "other" Lee Sun. Kim Myung-soo, better known as L from the K-pop group INFINITE, took on the role of the commoner who shares the Prince's name. This was a massive turning point for him. Before this, he was often labeled as just another "acting idol"—a singer who gets roles because of his looks rather than his talent.
He shut a lot of people up with this one.
His character arc is actually the most tragic. He starts as a brilliant peasant who isn't allowed to learn because of his status. He ends up becoming a puppet king, sitting on a throne that doesn't belong to him, consumed by an inferiority complex and a desperate love for Ga-eun. His descent into obsession is uncomfortable to watch, which is exactly why it was a good performance. He made you pity him and want to punch him at the same time. That’s hard to pull off.
The villains you loved to hate
A drama is only as good as its bad guy. Heo Joon-ho played Dae-mok, the head of the Pyunsoohwe. He’s the one controlling the water. He's the one making the King kneel.
Heo Joon-ho is a legend in the industry. He doesn’t play "cartoon" evil. He plays "I actually believe I am doing the right thing for the world" evil. It’s terrifying. Every time he sat across from the Prince, the tension was thick enough to cut with a sword. He represents the corruption of the old guard, the idea that power belongs to those who can seize it, not those born to it.
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Then there’s Yoon So-hee as Kim Hwa-gun, Dae-mok’s granddaughter. Honestly? A lot of fans actually liked her more than the main female lead. She’s a tragic anti-hero. She loves the Prince so much she’s willing to betray her own family, her own blood, to keep him alive. Her ending is one of the most heartbreaking moments in the entire 40-episode (half-hour format) run.
Why this ensemble worked
The cast of Ruler Master of the Mask succeeded because it balanced two generations of Korean acting. You had the young trio—Yoo, Kim, and Kim—bringing the raw, emotional energy of youth. Surrounding them were veterans like Park Chul-min (who provided the much-needed "funny uncle" comic relief as Woo-bo) and Kim Sun-kyung as the Queen Consort.
It wasn't just about the leads. Even the supporting roles, like the Prince’s loyal bodyguard Chung-woon (played by Shin Hyun-soo), felt essential. They built a world that felt lived-in. When a character died, it didn't just feel like a plot point. It felt like a loss.
The lingering legacy of the cast
Where are they now? This is the fun part.
- Yoo Seung-ho went on to do I'm Not a Robot, which became an international cult favorite.
- Kim So-hyun solidified her "Queen of Saeguks" title with River Where the Moon Rises.
- Kim Myung-soo transitioned into leading man roles in Angel's Last Mission: Love and Numbers.
- Shin Hyun-soo became a staple in the Welcome to Waikiki series and the recent Duty After School.
The show consistently topped the ratings during its airtime, hovering around 12-15% in South Korea. That’s huge for a mid-week drama. It proved that the "masked hero" trope still had legs if you put the right people behind the masks.
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What you should do next if you're a fan
If you've already finished the show and you're missing that specific vibe, there are a few things you can do to scratch the itch.
First, go watch the behind-the-scenes "Making Of" clips on YouTube. The contrast between the heavy, depressing scenes and the cast laughing between takes is wild. Yoo Seung-ho and Kim So-hyun have this adorable, respectful sibling energy off-camera that makes their on-screen romance even more impressive.
Second, check out the soundtrack. The OST for Ruler is top-tier. Bol4's "From the First Time" and Yang Yo-seob's "The Man That Couldn't Confess" are essential for any K-drama playlist. They capture that "longing in a rainy palace" feeling perfectly.
Finally, if you want to see the leads in something completely different, watch I'm Not a Robot for Yoo Seung-ho or Bring It On, Ghost for Kim So-hyun. It’s a great way to see how much they’ve grown since the days of the Pyunsoohwe and the water wars of Joseon.
The cast of Ruler Master of the Mask wasn't just a group of actors doing a job. They were a perfectly timed collection of talent that defined a specific era of historical television. Whether you came for the politics or stayed for the heartbreak, these performers made the story stick.