Seriously!

Think it’s because he’s already seen many of her lives before, he believes that she’s pulled in by her destiny and can’t escape from it but he wants her to live because every time she goes through her “destiny” she dies. Also, most of the time these couples always have some kind of problem with communication which I don’t understand either. Maybe in one of her past lifes, he has told her before but that obviously ended with her life in the cycle again, dunno maybe I’m giving him to much benefit of doubt but when you see your love always die right in front of you when you try you hardest to save them, there’s bound be to a lot of trauma. Also I think even if she did know, she’s the type to go through with it anyways. Even if she knows she’s gonna die, she’s gonna risk her life for the greater good like the hero she’s claimed to be. It’ll be different this time otherwise there wouldn’t be a story to move forward with if it just ends the same as her past lives.

I do think he needs to tell her everything but idk if she'd change her mind after he tells her everything. She seems quite stubborn. She seems like the fl that'd say" if its my destiny then i should follow it. I'll be careful i promise" then she gets into trouble, saved by the ml. Maybe she'd discuss some things n strategise a bit with the ml but in the end she gonna do sth stupid #－.－)

I think even Thessar tell her everything, she will just get more eager to help people and sacrificed herself, i dont know,when she said she want to help people and willing to get separated with Thessar she is just selfish. She is not considerate Thessar feeling at all.