I guess now is a good time to say this. It is well known that there are many interpretations of vampires. Bethany is not the classical definition. This interpretation is that of vampires being a separate species, and not "undead" (as in they reproduce like any other creature, biting does not make others vampires/servants). Their immortality is purely reliant on blood and if they do not drink blood they begin to age like humans do. A side factor of this particular variant is that they prefer blood of a particular psyche (a liar, depression, angst, joy, etc.), Bethany prefers arrogance, upon feeding they remove this aspect of a person i.e. a liar will become a more honest person as a result of being fed upon.
The reason she could detect Wilo was because she could see her bodiless psyche