- Kiku was not born a vampire, vampires don't reproduce that way.
- Nazuna was also born from a human, her mother was a vampire but her father was human. Everyone treats her as a vampire because she's the daughter of a vampire and able to use the same powers as a vampire, but she lacks at least half of their normal weaknesses. (She's a dhampir, but apparently no one knows this term in this series)
- Kiku died right at dawn as the sun rose, it is entirely possible that it was the sun that killed her. Supporting this idea, she and the guy both turned to ash at the same time, there was no scene of her turning to dust because she turned human and the years caught up then the guy burning to ash in the sun, they both just turned into ash as the sun rose.
On a related note, even though we have proof that sunlight can be almost instantly fatal to vampires, Nazuna doesn't seem to care about it very much even while the other vampires seem to be terrified of the sun. (We never see them out during the day, but we've seen Nazuna strolling around with nothing more than a parasol) She also lacks any attachment to her time as a human because never was human, which is the main weakness of vampires, and doesn't seem to have the same strength or range of abilities (eg, she doesn't seem to be able to charm people the way Haruka and Kiku could) so even if falling in love was fatal for the other vampires, it's almost certainly fine for Nazuna.
i mean kiku was in the sun before, the same way nazuna has (with an umbrella, though nazuna and others have also been in the sun without) as have most of the other vampire characters at different points.
the only vampires that have died instantly in the sun are vampires who are exposed to the sun, and their 'weakness' (which is basically their most loved <thing>) at the same time. Kiku didn't become human, it's kind of been implied that it's more like vampires can't experience love in the same way they could as humans. But if they do, it seems to me that it essentially overwrote whatever her most precious <thing> was in her human life. A vampires weakness is love, basically.
I also am kind of assuming that Nazuna's mother Haru died after giving birth because of her love for her child. She experienced human love again, through Nazuna.
thats what i'm guessing, anyway. I'm not sure any of these rules apply to nazuna though, and I'm pretty sure they don't. It's been pretty obvious that Kou loves nazuna for a long time, and him being a 'half' vampire to me seems more likely to be a result of Nazuna being essentially a half vampire herself.