Is Kou forgetting or not aware that he will die unless he becomes a vampire?
Also, I feel like Nazuna was genuinely hurt by him not wanting to become her underling. She says the past ten years have been boring, but she can't lie, the nights that she has spent with Kou have been enjoyable.
Now I'm interested in her backstory, and who she fell in love with that turned her. He or she is probably the reason why she has such an aversion to loving someone.
And Kou doesn't think that he and Nazuna aren't friends? How stunted is this kid? They aren't lovers or anything despite the sensual nature of the blood sucking ritual but how can he think that they aren't friends at this point? 
Love this manga. Thanks for translating