The canonical names started as Vampire and Human, but I ended up coming up with different V and H names cause that felt too weird. Their canonical names are Valen and Haley!
As far as why Haley falls prey to Valen so easily, I think that's up to you to interpret. My personal interpretation leans toward a combination of being overwhelmed by life and having to be the caretaker in previous relationships.
Thank you for thinking so much about the VN and its characters, it means a lot! <3