Nobody Knows
아무도 모른다 (original)
Choi Su Jung, a 16-year-old high school student, is brutally murdered by a serial killer. Her friend Young Jin gets to take a phone call from the killer and says that she’ll find and kill him. Over two decades later, Young Jin has become a police officer and still desperately tries to find the serial killer. She has a young, middle-school-student neighbor Eun Ho. He gets shocked by what’s inside Young Jin’s room when he comes over to her place.
Starring Kim Seo-hyung, Ryu Deok-hwan, Park Hoon, Ahn Ji-ho, Jo Han-chul, Seo Young-joo, Min Jin-woong, Moon Sung-keun, Park Chul-min, Baek Soo-jang
Watch "Nobody Knows" on Rakuten Viki released in 2020. It's a crime and drama show, and has a rating of 8.1 out of 10 from Imdb which is very high for this kind of show, and is currently available to watch for free.