THE MAW
The Maw was created centuries ago by a surnatural being, possibly a witch who fed on vital energy or souls. Mostly like the yokai city in Spirited Away, its only goal is to lure clients with its rich, comfortable services.
The Maw may have started as a simple boat with little structure, and quickly developed thanks to the guests' money and...'by-products' (I'll detail that later).
The Maw has been run by different witches, maybe 5 so far with Six being the new one (I'll also develop that later). Thanks to her powers, the original witch (One ? ) may have created her servants (The Janitor, the Twin Chiefs...) from the corpses of Guests she absorbed the souls of.
The food served to the Guests basically consists in fish and meat, with the most delicate and priced meat being...children meat. It's unknown where the kids come from: maybe they were the Guests' children, or maybe they were kidnaped thanks to a complex and sick network of kidnaping coming from all over the world. Some meat loafs seem too big to be children, though, so some meat may actually come from the Guests themselves, as we know they never return from the Maw...That Guests meat could be used to help the Guests become bigger and fatter, because the children are so skinny and malnourished I truly doubt their meat would be really nutritive...
Therefore, even if the Maw looks like a pleasure resort from the outside, it's actually more a concentration camp and slaughterhouse...
THE LADY
The Lady is a witch who feeds mostly on vital energy and/or souls. She wears a mask to hide her old, malformed face. It is unknown weither or not she was born this way. I personally think her 'ugliness' is a sign of growing old, and that as time goes she needs to feed on more and more souls to rewind the effects of aging ; until it doesn't work anymore. That could explain all the hidden portraits everywhere in her residence : she may refuse to be reminded of her now fainted beauty.
There are portraits of a very similar looking woman which aren't hidden, however. So maybe that woman isn't her but was a previous witch she'd want to look like.
The Lady lives surrended by puppets who strangely look like petrified children. Her residence is protected by shadow children who only appear in the dark. Maybe those puppets once were real children she did like, or maybe children she conceived herself and changed into puppets after she ripped their souls apart and made shadows out of them.
The Lady has a particular hatred to mirrors. Most mirrors in her residence are broken, except two, which are both hidden. One of them is in a far away room and she can be seen crying while looking at it. Another one is the oval mirror Six defeats her with. It's interesting to note that a similar mirror, a broken one, can be seen in the DLC : maybe it was the mirror the current Lady (Five ?) used to defeat her predecessor (Four ?)
Since Six defeats the Lady with the mirror and then feeds on her by sucking her powers, that may make sense that the mirror is a magic one destined to be used by the next witch in the line...
A new mirror may magically appear at a random place everytime a new Lady arises. Or maybe the oval mirror is actually a strong magical artefact that gives the Lady her powers but is the only object capable of destroying her - like the Dark One's dagger in the TV show Once Upon a Time.
THE HANGING MAN AND THE ALL-SEING EYES
I have a very personal theory about who The Hanging Man was, and how he could be linked to both Six and the all-seeing eye machineries (those machines with a button, that show like still camera inputs). From the machines' size and the location of the first one, I wonder if they wouldn't be used by a normal sized creature - like a normal human with long legs. Maybe there once was a servant whose job was to watch over the Maw with these machines. You can notice that the first machine ends up showing the oval mirror used to defeat the Lady - so maybe those eyes are not meant to watch over regular children, but to watch over the potential new Lady and prevent her from reaching the mirror.
That servant, who I call the Watcher, may have been the Hanging Man. He may have noticed signs that Six was probably going to be the next Lady (like her hunger crisis) and decided to take her away from the nursery and keep her in a room near his quarters to prevent her from becoming the next Lady. He didn't kill her, though, maybe because he felt attached to her, or because he hoped he could change her nature and make her a normal child. When he discovered (how ? I have no idea yet) that his mission failed and that she would never become normal, he decided to end his own life.
SIX
I'm pretty convinced her 'name' is totally linked to her function : since she defeated the Lady and sucked her powers, I'm rather certain she's the new Lady, the sixth one since the Maw was created.
Another element in favour of this theory is that she has regular hunger crisis, that evolve through the game from a need for food to a need for absorbing life (maybe soul) : after eating bread and rather normal food, Six quickly experiences hunger again and decided to eat a rat alive. What could have seemed like an act of despair at first is later revealed to be a whole different sort of act when she decided to eat a poor Nome (actually the Runaway Kid) instead of the sausage he was handing to her...and ultimately, the Lady herself.
Maybe what brought Six to the Lady was not the desire to be free, but some irresistible hunger to eat her powers and soul.
As she walks in one of the dining rooms after defeating the Lady, she doesn't try to restrain her powers at all, and absorbs the Guests' vital energy/souls, becoming the very nightmare she seemed to try to escape at first. In the last post-credits images, she's shown waiting at the surface, near the Maw's door, but I'm pretty sure she's not waiting for being rescued - I actually think she's waiting for the next boat to arrive (the honk heard at the end) so she can suck the new Guests' vital energy and get stronger for her new Lady job to begin...If her goal was to free the other children, shouldn't she come back to the nursery, instead of waiting alone by the door ?
We know nothing about her origins, but that's interesting to note she doesn't wake up in a nursery, on contrary to the Runaway Kid. So my theory is that she was taken away from the nursery by the Hanging Man (the one I call the Watcher) and kept in isolation in a desperate attempt to prevent her from becoming the new Lady. I think she originally comes from the nursery, though, as she seemed to know the way to it and out of it - unlike the Runaway Kid who used to follow another escaped child.
