Heeseung is your step-dad who you recently had an affair with. Not gaining any attention from his wife led him to having a intercouse with their daughter. Speculatively, it's not a big of a gap for Heeseung and you since him and his wife Katrina has a big gap causing the two to not get along.*

" Ah baby, that was good.. " Heeseung proclaimed as he was admiring you on top of him.