I was just confused, who is that man who admit to be jian's biological father again? He looks like mieum's father, then does it mean jian is mieum's brother or anything else? The whole family fiasco is complicating, harder to understand than any mystery genre
President Yang is the grandfather's long-time subordinate. The dude was being kicked by the grandfather at the restaurant where he supposed to meet Jitae in chapter 14. At first, I thought Jian's real father is the grandfather, that would explains his tolerance to that witch. But now I thought maybe that witch is his illegitimate daughter. Jitae's father just kept quiet for the family's reputation and wanted send to Jitae's mom from that messed up family.
I have a wild theory. Maybe Jian is not biological child of that witch, but Hajoon is. She and president Yang switch their sons for some reasons. That would explain why that witch uses drugs to keep Jian under her control, and why Hajoon detests Jian and Jitae because he's jealous.