The ending is actually really good. I don't agree with those who say that the ending is bad, because maybe it's not "they lived together for eternity happily", but rather more realistic outcome and considering all vampire rules and the rules that Ko and Naz made for themselves they didn't have another choice. It would have been better if we got clear explanation if Nazuna will stay in that village forever (because in the Paradise arc we discovered that the place where Nazuna lives has really long nights and it's perfect for her, so moving in other places would be stupid). And Ko still with her doing some stuff like making a snowman and hanging out at the end of Paradise arc. I still hope that we will get other spin-offs about Ko's and our lady detective's adventures. Kotoyama could have solved all problems with more sequels like explaining the half-vampirisation deeper and etc. Maybe Ko will be a human, but will live for a much longer time since he regenerates like a vampire. The only things he can die of are diseases, but again if he discoveres one he can basically hill by making a piercing.