I feel like people are getting way to hung up on the whole Vampire reasonings/origins behind Kou and Nazuna. I literally think its as simple as, Nazuna born from a human x vampire is a vampire. However, whoever she turns only becomes a half vampire due to her origin. Bam thats it. Don't think the author ever wanted to go any deeper than that, and constantly reminded us throughout the whole series that vampires themselves barely know shit about vampires. So everyone in the story is just as clueless as us the readers. Which is why they didn't want to risk potentially life threatening stuff, in fear of doing harm to the other person or themselves.
As for the ending, it was fine. Sort of what i guessed might happen. I'm cool with open endings if done right, which for this manga seems appropriate. Seems like they were trying to please everyone which is why some people might feel mehh about it. Others like the hardcore rom-com andies are absolutely seething, only happily ever after's can soothe their souls. I like bittersweet endings as well, if anything the leave a bigger emotional impact. Plus this felt more like a coming of age story more so than just a romance story. We got to see Kou grow and mature throughout the manga. Coming from a lost kid, struggling to understand himself and others to someone who now has a better understanding of himself and others, and made a bunch of friends along the way. Love just happened to be one of those things, along with the pain it brings. Now look at him, he's a straight up G. Girls falling for him left and right, has a sweet detective gig set up for him and he has his half vampire powers without the drawbacks. He went from a loner edgy doomer to a confident semi-gigachad status highschooler.
One major nitpick I have with the ending is the blood bag. Why the hell is this being brought up now? The author just opened a whole variety of questions that goes against the conflict of them not being able to be together in the first place. She literally just tried to succ him and he's like I learned some karate, btw "hErE's a bLoOdbAg, iT's MiNe". You telling me, bro is so desperate that he traveled to some remote snow place in Siberia and is willing to risk potentially killing the person he loves with his blood bag? Like yeah i get it, love makes you do crazy things, trust me i get that. But I wouldn't want to pose the risk to her unless she herself had the idea or wanted it. Unless we are to assume its perfectly fine since its in a bag and not from his body directly? Idk it felt completely unnecessary and only added questions to the story and the rules. You could remove that whole bit and leave the reader with less questions and would be just fine, more open ended if anything. I guess i'll end my rant.