I am pretty sure later it will be revealed like if vampire feed on men that she love she become human and instantly die and also as mentioned a little bit earlier person on whom she fed die too, maybe it some kind of weird metaphor or Romeo and Juliet homage, either way as it said in dialog...
This arc was horrible, but author fixed it a little bit at least for me in the end, also good thing that they both will not return, I hope other annoying cookie cutter characters that comes with kiku will be just swipe under that rag and forgotten. And I pray to god that next arc will not be...
I am pretty sure that in this arc Kiku mentioned that she needs more and more feeding implying that one day she physically couldn't procure that much of blood and eventually die from starvation or will be hunted. Maybe Iam read to much to that, bit I understand it like this.