There is a hidden eldest son. Oh Seung-yoon is the biological son of Kim Eun-young. He was left in the care of his maternal grandmother at age 2 and lived alone since middle school at age 15. He entered the police force in his 20s and has been a detective for 3-4 years. He is currently on a joint investigation mission. Kim Eun-young and Oh Jeong-taek do not realize he is their biological son, though he slightly resembles his father, Oh Jeong-taek. He has been active in the field for a long time.

(Glaring sharply at the CCTV screen, then stepping in front of the team leader) "A perimeter sweep? I'm the one who stayed up all night guarding the alleys because there's a high chance he'll escape through this passage. Why are you suddenly sidelining me? I've survived on my own since middle school, grit my teeth through everything. I worked my tail off to become a detective who pulls his weight at the agency! Why are you treating me like some incompetent rookie? If a detective is afraid of getting hurt, they should hand in their badge. I'm going to fulfill my duty. Detective Oh Seung-yoon, moving in."

(Urgent, blocking Detective Oh's path with her body while holding her pistol) "Detective Oh Seung-yoon! What do you think you're doing? Fall back immediately! This is my department's case. The Metropolitan Unit goes in first, so the Detective Division stays on the perimeter. Detective Oh, especially you—don't you dare move from this spot. That's an order! No arguments! It's an order from the scene commander, so just get back! It's not because I dislike you... it's because... because you can't get hurt... (Screaming at his retreating back as she hits the radio) Captain Oh Jeong-taek! Jeong-taek! Seung-yoon... Seung-yoon went inside alone! Go after him now! Please, protect my son!!"

(Setting the National Forensic Service envelope heavily on the desk with reddened eyes) "I felt something strange, wondering how someone could even share such peculiar habits... but blood doesn't lie. That kid... he endured all that lonely time doing this dangerous detective work, never knowing his parents were right beside him. Seung-yoon. Could you read this document before we eat? We are your parents. When you were two... we left you with your grandmother for a short while because of extreme poverty and an investigation, and then we lost you... We are your pathetic mother and father. The tears you shed, the lonely hours you spent alone... I will spend the rest of my life making it up to you."

During their middle, high school, and college years, Yoo Ji-ho, Yoon Bo-mi, Lee Seok-hyun, Lee Yeon-hee, Kang Young-tae, Kim Jun-ho, Oh Seung-yoon, and Kim Min-seo were seniors and juniors to one another.