There are so many interesting things about this series. One of them is the contrast made with the characters.
Mahiru and Kiku had a sort of odd relationship. He fell in love with her whilst she was actively trying to fall for him. They spent a significant amount of time together until she finally understood her feelings for him. However, in that approach prior she fell for him, she told him already she would die soon. There was a finality to their relationship, where she was slowly dying with or without falling in love, but she wanted to die human.
On the other hand, Mahiru began craving a place he could belong to. His family was broken, he had a secret rivalry with Kou (even though he was his best friend) and when Kiku comes around, she is the perfect escape to everything causing him pain. She guided him to break up with his family, she guided him to break up the relationship with his friends... She guided him to isolate to the point he would only depend on her.
Now, if we look at Kou and Nazuna, the story is very different.
Kou was bored, often feeling he doesn't quite fit the role others expect out of him. Nazuna was the same, feeling she was a little of a renegade vampire, not interested in love and just trying to spend her time on fun stuff.
Kou comes from a home with a hardworking mom, that gives him a lot of liberties because she knows he will come out fine in the end, but she is still loving with her son.
Nazuna has never tried to break Kou's relationships, instead she has engaged her friends and actively get involved with them.
Her interest is in Kou's development to eventually make him her underling.
This interest and active involvement lead her to understand Kou's feelings, and eventually fall for him.
This relationship doesn't have "finality" set in stone like Kiku and Mahiru's relationship. It is filled with doubts of things that might be and fears to overcome.
I am not sure the "when a vampire falls in love, time catches up to them and they die due to accumulated time:... because Nazuna's mom fell in love with a human, she got pregnant and all prior her death (if it happened), whilst Kiku's love appeared to be aggressively harsh, though she was already in the process of dying.
I really don't know the direction this story is going to go and how it will end. Will it be tragic, revealing to us that Nazuna decides to make Kou a vampire whilst she sacrifice her immortality, leaving Kou by himself?
Will we find out Nazuna can make Kou into a vampire because he is halfway there already and it won't affect them?
Will we have a long series of plot twists, unveiling the truth behind "vampires who fell in love with humans become humans"?