I really thought the mc is a vampire? XD

I think the FL was first a vampire, and then she transferred her immortality to the ML to save him, which is why now she is 'almost' mortal(she still has vampire blood in her but i guess it isn't strong) and ML is a vampire now

no she wasn't vampire, she was someone whose family died, and she was weak, so out out of nowhere some person with immortality came to her and gave her their immortality, then dia gave her immortality to caire so, what you said is right except for the fact that she's a vampire