let me preface this with: I don't know a single person satisfied with the ending. Also I'm not sure how much of what I'm about to say is already covered in S1 so there might be S2 spoilers here anyway.

If a human doesn't turn into a Vampire within a year they die.

If a vampire sucks the blood of a human they love they turn mortal. Nazuna is a rare born vampire, she didn't turn into one, no one knows whether this rule applies to her.

To turn into a vampire you must be in love with the vampire biting you. At the very end Ko does fall for her. He knows it, she knows it, everyone else knows it. She refuses to suck his blood due to aforementioned unknown. Nothing happens. Like legit nothing.

Ko doesn't turn into a vampire. Ko doesn't die either. For some fucked up reason he just stays a half-vampire without any repercussions.

Nazuna doesn't die either because, as mentioned, she refuses to suck his blood. Instead she just kinda fucks off with Ko at the end seemingly intent on trying to find her. That's it, the end.