A big part of why Four Hands, Two Sonatas worked for me was the cast. Even the supporting characters feel like full people, with their own regrets and blind spots. Here’s who plays who, where you might have seen them before, and my honest take on each performance.
Song Kang as Kang Bi-o
Bi-o is the perfectionist: a piano prodigy shaped by years of relentless practice, a demanding mother, and a grandfather who won’t approve of him.
Song Kang is known as the “Son of Netflix,” since most of his shows stream there, including Love Alarm, Sweet Home, and My Demon. But he’s never done anything quite like this. Outside of romance and Sweet Home’s kind of genre, this is new territory for him, and he’s natural from start to finish. His line delivery has noticeably improved too.
The piano scenes are where he really got me, especially in Episode 5, at the final of an international competition in Prague, where he keeps playing as his fingers bleed onto the keys. And yes, I’ll say it: he’s never looked more handsome.
Lee Jun-young as Choi Jeong-yo
Jeong-yo is the natural genius, born poor and raised by an alcoholic father, with a talent nobody taught him.
I honestly didn’t know much about Lee Jun-young before this. He’s also a rapper and singer, and his acting credits include When Life Gives You Tangerines, Love and Leashes, and Melo Movie. By the end of the show, I was a fan. He’s a skilled actor and a great partner for Song Kang. What impressed me most is how he kept Jeong-yo’s humanity and optimism intact. Jeong-yo just keeps pushing forward, through everything, without ever losing his integrity.
Jang Gyu-ri as Hong Jae-in
Jae-in is a viola student with an exceptional ear for music. She’s the one who first hears Jeong-yo play and brings him to the school, and she’s loved Bi-o since they were kids. You might know Jang Gyu-ri from Cheer Up or The Player 2.
She’s a great supporting presence, and in a way, she’s what makes the bromance shine. I mostly felt for her, though. Her whole life seems to revolve around these two boys, and Bi-o never loves her back.
Yoon Se-ah as Kang Ji-hye (Bi-o’s mother)
Bi-o’s mother is obsessive, controlling, and desperate for her son to win. I ended up both liking her and pitying her. Yoon Se-ah plays a mother who loves her son too much, while she’s still craving love and acceptance from her own father. That second layer is what keeps her from being a simple villain.
Jung Jin-young as Kang Seung-woon (Bi-o’s grandfather)
The grandfather is a legendary figure whose philosophy of music and art shapes the whole family, and that philosophy comes at a cost. He’s lived a mostly unhappy, tormented life, and it shows. It’s hard to watch him reject Bi-o and his playing, while clearly still deeply fond of his daughter. Jung Jin-young, a veteran actor, makes every scene he’s in heavier in the best way.
Kim Joo-hun as Do Hyeon-soo (the teacher)
Do Hyeon-soo is the mentor every student deserves. He recognizes Jeong-yo’s genius early, supports Bi-o even when he can see his limits, and has a soft spot for Bi-o’s mother. Kim Joo-hun plays him with a genuinely good heart, and he’s a quiet anchor in a show full of storms.
⚠️ Spoilers below
The grandfather’s final scenes hit me harder than I expected. Before he dies, he apologizes to Bi-o, admitting he was the one who caged his grandson’s potential. He also apologizes to Jeong-yo on behalf of the daughter he raised, saying he’s sorry for how she turned out. Then he asks Bi-o to play Beethoven’s “Moonlight” Sonata for him and to leave him alone with the music for a minute. Bi-o steps out, and he passes away. It’s quiet and devastating, and it’s one of the best scenes in the show.