They stage the masquerade (Part I), more or less(Part II), and they have reasons (Part III), and they take on the role (Part IV), and they have means to stage it (Part V) -- so who is the object of all this effort? Whom are they protecting?
Why, all the magical beings of the world. At least in their magical side. That is, after all, the function of the plot device.
Indeed, I suspect that many, many, many tales start with the magical beings and work out the masquerade to suit. Certainly the interactions are profound.
If you are protecting dumb beasts that happen to be magical -- the phoenix, the unicorn, the gryphon -- they had better live in the wilderness, unless you have a really impressive zoo. Then, most masquerades have such creatures as, at most, peripheral. Or, frequently enough, intelligent enough to speak, and thus be part of the masquerade as more than the protectorate, or at least be reasoned with.
When dealing with intelligent creatures, the most important question is how much do they want, or need, to interact with mundanes. At one extreme, vampires may be absolutely dependent on human blood. At the other, unicorns notoriously don't want to associate with anyone.
Alas (from the point of view of those maintaining the masquerade), between what they want and what they need, the one that rules is the one that requires more contact with humans.
A pixie may have no need at all to pixie-lead people, but refuse to stop barring compulsion. A werewolf may pick fights out of bravado. Jenny Greentooth may continue to drown anyone she gets her green fingers on.
Conversely, a ghost, or a brownie, may be tied to a human habitation. A vampire may require fresh human blood. Mermaids may need to spy out ship activity.
Still, which creatures are behind the masquerade may influence many things about it.
An insular group of fauns and nymphs, living in their forest, piping and dancing between feasting and drinking -- perhaps they have a cornucopia? -- may not care to leave. Perhaps they prank hikers -- perhaps viciously -- but they are easier to hide. Their biggest problem may be that no one else would do it for them, and they have no wish to keep up the level of surveillance necessary to catch any dangers they are in. Due to their insularity, they are also the most likely to engage in ruthless enforcement while not otherwise acting ruthlessly, as long as the enforcement affects those who are outside their group.
Then, the question is whether such insular groups are those concealed. Perhaps the development of modern technology has led to a situation where sensible magical beings fear the fate of the dodo and the passenger pigeon and the dragon -- which would go and demand the princess and get a knight to slay it.
Unfortunately, it's the others who are the problem. That all rash magical beings would wipe themselves out is as unreasonable as all rash mundanes doing the same. Then the others may matter in what steps they find acceptable to curb the rash ones.
The wizards who guard the masquerade may wish to contain all wizards in safe, rural villages where everyone is in on the masquerade, where the quiet makes study easier, and the distance makes tests more concealable. Still, the wizards who insist on the bright lights of the big city are, well, wizards. Unless the world-building is careful, they have the power to resist. Particularly if other wizards support their freedom. They may even regard it as the duty of those upholding the masquerade to handle all of it, and any duties for the rest of them as imposition.
How this works out depends on the details of the masquerade. Do you want to cover up the magical beings entirely, or just the magic? Are the characters happy if they attribute the murders to wild wolves instead of a werewolf? Do they execute the killer, and if so, for murder or for violating the masquerade? Do the wizards and officials who are in charge of the masquerade grimly accept that the werewolf tribes will execute murderers even if for the wrong reason?
This will greatly affect the tone of the tale. Not only what are those upholding the masquerade are willing to do to protect it, but what those protected are willing to put up with. The more isolated the protected are from those outside, the less likely they are to care about them. (Still less, of course, if the masquerade is to protect the mundanes from them.)
Then, this may be less of a factor. Are the protected the same sorts of beings as their protectors? Are their powers greater or less? (Their effective powers. If the protectors have special magical objects that make them more powerful from being the weakest, they are still more powerful.) Are they more or less in number? A great number not only makes it harder to hide so many, it also makes them more powerful -- if only in effect, since so many can attack at once.
The protected's particular magic is also a factor. If the fae can hide themselves with illusions even from their protectors, if the vampires can charm the wizards, that is effective power even if the wizards could blast them with fireballs.
If the protected are more powerful, the masquerade can only work if the vast majority of them are acquiescent in the matter. Or perhaps even cooperate to a degree. All the more in that the protectors have to keep watch everywhere.
If the protected are less powerful, the masquerade will be simpler. Possibly enough to let it drop to the background.(Digression on this)
Most stories where the masquerade is the foreground, those who try to break the masquerade are just enough more powerful to make keeping them in a challenge to the protectors, for obvious conflict benefits. Their powers are often different, to a greater or lesser extent, so that the protectors must use great skill and strategy to use their powers effectively and overpower the other side's more brutal force abilities.
With this, I conclude this series. It is far from exhaustive on the issues, but I hope I hit many useful highlights.
"Protect the Pixies" by preventing the "mudanes" from living in areas that Pixies like.
Of course, what happens if Pixies move into human areas?
Do the Protectorate move the humans out of their homes? [Sarcastic Grin]
The talk about werewolves reminds me of some comments from the Dark Osprey book on Werewolves. Namely how urban werewolf packs often get on the good side of the human neighbors because when they move in, the rates of violent crime and large/dangerous urban predator infestation (coyotes, etc.) in the neighborhood drop severely.
The problem comes when you have two werewolf packs who want the same territory, especially if one consists of of lawbreaking 'fangbangers' and the others are more peaceful 'Baltos' or 'LAWYs' (Law-Abiding Werewolf Yuppies). Especially when both sides start calling in support from other packs.