Diamond reassures the human that she and Silver are not afraid to dominate adults with both their status and power, literal physical power and can demand whatever they want from them at the the sound of their loud rambunctious voices, mostly Diamond's loud voice. These two are also aware that many chose to serve under them just to be abused by them. The girls wanted a human at their correct size to do the same thing to them so they planned the whole thing to bring in one that was already aware of this world and won't resist. Diamond originally wanted a buff male, but had to act quickly and found a scrawny one instead. It would have been more problematic to bring in someone never being here. However, Diamond wanted to try something before she has him do any work...