It's by Ailee, and it's Korean characters translated by Google translator into English as "Just Staring." She also sings a lovely "Heart is you" in another fabulous series, Memories of the Alhambra. Awesome addition to both.
It turns out that anyone who asks and discovers AILEE will find an enormous talent that has a sensational portfolio of original soundtrack OST in her resume that opens up an entire world of simply outstanding romantic film and television series of Korean work. I discovered that AILEE is herself alone a gigantic star and superb musical talent. I've only spent hours and hours listening to her performances. She's a legitimate DIVA. You'll hear her songs in many TEN worthy Korean films and tv series. Netflix discovered that - and they hit a home run.