From what I understand

Reading the story in English, I think what is implied about the Father is that he was lonely and jealous that he could not have a kin. In the chapter where he seduces Gunchan, in his thoughts he says, "You should be out of your mind by now".

He knew he liked Gunchan just like all vampires instinctively recognize potential kin. However, he got angry at Minhoon and Gunchan defying him so went ahead and bit Gunchan. He had no idea if it would drive Gunchan insane or not, but did so anyway.

It seems very implied that he did try to take kin like other vampires but they either died quickly or went insane. He stopped trying 500 years ago. Father is way too strong and being his kin seems to erode the sanity and physical health of any human he attempts to turn.

To his surprise (and delight), Gunchan kept his sanity. Now he likely wishes his dominance affected Gunchan a little (like assimilation), but it probably makes him want Gunchan more since Gunchan can defy Father.

Father might be the most powerful of the vampires, but the price seems to be an eternity alone since he likely couldn't keep his kin sane or alive for long (in vampire terms).

Once he realized he finally had a kin who could withstand a bond with him, he started softening his stance on Minhoon and his son and the whole system right away. Notice how it's even mentioned in that meeting they all have. Edric and Sungreul suspect that father is willing to go with Minhoon's terms mainly because of Gunchan.

Father is like, "Wait, I got a kin? I really get one I can keep? **ck all you bitches. Yeah, sure, Min. Have fun with the kin system. Later ~" *off to follow Gunchan around*

Basically, Father was bitter that everyone but him got a kin. So he made a system where no one got to enjoy having a kin. Now that he's got a kin. Everyone can have a kin after all.

He is never going to let Gunchan go. Gunchan seems rather chill about the whole affair. But don't underestimate him, he seems willing and happy to call out Father.