Wild speculation since the Official online raws are up to 37, while the translation is up to 19, but I'm sure it's going to be revealed that she's mute because of some trauma and the cop reminds her of or maybe even is the person that saved her before. Also, assuming she's not being dishonest, she's 18 and can do what she wants, but more than that, the wild age gap is the whole reason the inspector is trying to keep her at a distance and dissuade her from loving him. The fact that the relationship is a little coercive, but it's the young one who is doing the coercion is the whole joke.