Responses
It's really dodgy. He hides behind her because she's the strongest leader, but (and this is my own theory) I think he's envious as hell that Heeseong accomplished what he wanted. He couldn't get Chiyoung or weaken his power as the enforcer, so he hides behind the so-called strength of the sister.
So the ex guy, is he really only for the revenge or he liked the ML and does he really like the sister?