@Echonic In order for mermaids in this world to breathe properly out of water their gills have to close up tight in order to properly move air in and out of their lungs. This girl's aren't closing so there's some sort of problem that he's going to help her with.
@OniBarubary Per the laws of fictional writing, telling her to never let Monster-san find out about the human girl altered the future to guarantee that he would discover her.
@Qelix It could be a matter of being a pain to redraw, or the author might use language with a meaning that's difficult to convey properly in other languages.