I think I figured something out about this series. Kiku died from the sunlight but drinking the blood of someone you love and who loves you back makes you lose your memories of your past self so you die in a way. So when Haru drank the blood of someone she loved and was loved back, Haru “died” meaning she lost all her memories and she became someone else, which is Nazuna. Kou became a half vampire because if a vampire drinks from someone they love and isn’t loved back that’s what they become. If Nazuna drinks from kou she’ll lose all her memories and become someone else which is what happened to Haru. But I think if a half vampire like Kou drinks from Nazuna she’ll turn human, but she will lose all their memories also. So if half vampire who has mutual love with a vampire they can turn them back but will have no memory. So it solves the mystery of how you turn human and why vampires don’t know because they can’t remember how. I think first panel of the manga is actually Kou after he drank from Nazuna. But there’s still the mystery of who Haru fell in love with.