A backstory of their love story would be nice though. But I don't think they will show it, maybe some flashbacks.
I believe Saenare is dead, Tea Huk was petrified when Saya asked her if she was the one who killed Saenare.
Saya is not aware that he has a brother so that's why he don't care who the heck Eunseom is. I think he really do believe that he is a biological son of Tagon. Also, I think Saya used to be a nice guy but he snapped and turned cold and cunning when they murdered his first love.
I noticed that Saya and Tanya have something in common, and that is losing their first love (and reason to live), and that would serve as a connection between them. They both felt powerless then, and I believe they will team up to take revenge on those people that hurt them. I think that's the metaphor of the stone that Tanya's mother taught her, use the stone (tool/weapon) that is nearest to her, and that is Saya.
If you guys look deeper, Saya has a good heart. He was worried when he threatened Tanya (It was his way to make Tanya strong and think and not to wallow in grief just like what he did when Saenare died; and he keeps coming back and forth murmuring "What should I do? what if she really kill herself?" lol
and oh, I almost forgot. Tanya also cast a spell on Saya. hahaha I believe a love triangle would be possible in the future.