I just thought of something: What if Rumi's father isn't a demon, but it is none other than the Korean God of the Underworld himself, Yeomra. Someone who is above Gwi-Ma.
I didn't know about this divinity at first, but I just looked him up, and honestly, it would go hard! And it would make sense since Maggie Kang has revealed that the hunters, especially the first ones, are really tied to Korean shamanism, I think it would fit so much. Though I also like to believe that her father was just a regular hot demon who was able to fly under Gwi-ma's radar, who was much more negligent with his many, many demons back at the time. Then, somehow, he finds out that Rumi's dad married a human, but he doesn't know the wife was a demon hunter, and he doesn't know they had a daughter. So Celine accidentally killed Miyeong, and then Gwi-ma killed him. Or is keeping him captive to this day. Either way, I just want more about Miyeong and Rumi's dad, that considering what we got so far were clearly in love, and more about the Sunlight Sisters too