Dongbaek's mom reaction of extreme shock on seeing the attacker / Joker suggests Heung Shik's father could be someone she knew..maybe the abusive ex she ran away from when Dongbaek was a kid as shown in the flashback of Yong Shik's and Dongbaek's mother
Todays episode was just so so emotional and heartbreaking with all the revelations making Dongbaek realise how she and Pil-gu have always been watched over by her mom. This show keeps portraying even the obvious in such a heartrending way that you cant help but eventually root for each of the characters.