WAIT! Before I get tempted into demon lord x cute human wife, how old is the demon lord again? Isn't she like an adult in a child's body?
Yvonne does state that Faye is "five centuries old" in chapter 45. On the other hand, it's not clear what Faye's mental age is -- she certainly acts like a child, at least.
Just on the Cheekyness to be honest, lets not forget that she was supposed to be a potential love interest and since her original form is that of a teen/young adult it is fair to assume she would have a mental age accord to her body
But lets all be honest, im sure even Faye is aware that she has to wait xd