Isn't this reason a bit too presumptuous?
The Mirror Demon lady thought quickly for a moment and decided to keep her mouth shut.
Since telling the truth wouldn't have any effect, there was no need to speak it.
And there's no need to lie intentionally either, as lies will eventually be exposed one day.
Keeping silent is also a means of response—and it's a relatively smart means.
"...Not going to speak? Heh, what are you afraid of."
He looked at the silent person in the mirror, pursed his lips, shook his head, and sighed, "Really, giving you a chance and you don't use it."
However, even though the Mirror Demon lady didn't answer, Herbert could probably guess.
Because she's bored, she needs to find something to do.
How to put it?
What an odd one.
Compared to other Monster Girls who more or less had some negative emotions, this one's mindset is really quite... healthy.
A mindset full of bitterness and resentment?
No, no.
A yearning for freedom?
No, no.
