Mary is Guertena's idea of Alice.There are a few things in her situation similar to that of the dolls. She was left by her father as a painting when he died, she has her own world all to herself in the gallery, and she was quite willing to kill Garry for one person (As the dolls were supposed to kill each other for their father).
Ib's mother was one of Guertena's suitors.There is a painting of Ib's parents in the phantom art gallery called "Couple." Garry saw it alongside Ib and commented on how Ib resembled them quite a bit, so it couldn't have been one of Ib's conjured hallucinations in her distressed mind. Therefore, it is likely that Guertena met Ib's parents at some point in the past before he died. Guertena's Lady portraits depict Guertena's suitors, and the Lady in Red portraits in particular depict Ib's mother. The Lady in Red very closely resembles Ib's mother, with the same hair and eye color. Red eyes themselves are not exactly common, so who else could those portraits depict? (See the entry for Identical Strangers in the main page)
The entire twisted museum world is a Witch's barrier from Puella Magi Madoka Magica.A Witch manifested inside the museum while Ib and Garry were in it, and Ib and Garry were the unlucky ones to stumble into the barrier. The museum being a barrier would certainly explain the bizarre, incoherent, and disturbing scenery. All of the killer artwork would be the Witch's familiars. Furthermore, in Madoka Magica there was a Witch with an art theme (Izabel). It took on the form of the Arc de Triomphe and had the ability to talk. Therefore it's not a huge stretch to say that a Witch could take the form of a painting depicting an extremely lifelike yet imaginary girl named "Mary". Perhaps this painting-witch was once a fully-human Magical Girl herself. The rage and desires the painting expresses during the game stem from her father Weiss Guertena's actions during her human lifetime. Rather than provide her ongoing companionship and love like a father should have, Guertena neglected her emotionally in favor of pursuing his lucrative artist career. Not only did this leave the girl feeling extremely lonely, like she might as well have been nonexistent in father's eyes, but she also felt resentful towards the artwork Guertena favored over her. To fill the emotional void, instead of spending time with her busy father or exploring the outside world to befriend peers, the socially awkward girl lived her own imaginary world through the crayon drawings (Sketchbook World), toys (the creepy dolls), and storybooks (Carrie Careless and the Gallette des Rois) she created. In this world, she was omnipotent and would always have companionship. Considering that Witches' barriers are Mental Worlds depicting their troubled psychology during their human lives, the above back-story explains why the painting-witch's barrier is strongly Weiss Guertena-themed.
The paintings are based on real people who died.Apparently, it is possible to lose your memory and not regain it after you leave the art world. Also, people outside the world can have their memories changed like Ib's parents in Together Forever. Guertana didn't base his paintings on real people, but in the Portrait Forgotten ending Garry takes the place of the Hanged Man painting. Maybe the paintings ARE based on real people who died in the art world.
Garry is actually Gueterna. Gueterna also may have molested Mary and trapped her in a painted world.The game contains several references to both Alice in Wonderland and a girl who broke Gueterna's heart, almost to the point of obsession. Perhaps Gueterna knew an actual girl (Mary) onto whom he projected the personality of Alice. He became fast friends with this girl, which ultimately led to his molesting her. That's when she "broke his heart." She now hated him because of his actions. He knew about the painted world, so he painted her to trap her inside of it. He wanted to keep his deed a secret, and to preserve her for all eternity. The world itself is actually a manifestation of Mary's psyche. There are several references to this. Firstly, there are molestation references present. Hands reach out from the wall and grope at the player characters. There is one door that asks you to go into its mouth — another mouth on the wall wants to "eat your flower". In fact, the game's HP gauges are flowers — symbols typically associated with virginity, purity, and female coming of age. There are also signs that Mary has developed body image issues from the encounter with Gueterna, such as the deformed mannequins and doll heads which serve an antagonistic role in the game. The girls that crawl out of paintings could be seen as a literal cry for help-an echo of her ultimate goal of leaving Gueterna in the painting world. Not to mention the dolls everywhere. In fact, there are very few parts of the world that one would think were created by a grown man. The entirety of the alternate gallery looks almost completely like the work of a very disturbed young girl. The less solid aspect of this theory is the assertion that Garry is in fact Gueterna. After trapping Mary in the painting, he decided to send himself there as well (perhaps at the height of his regret) to spend eternity with Mary. Unfortunately, Mary still had not forgiven him, and her world was cruel and harsh to him as a result. This is the true reason why Garry looks so disheveled (his jacket and hair) when you first meet him; he is near dead from Mary's world. This is why Mary immediately dislikes Garry upon the two meeting. Mary spends the game trying to kill Gueterna and abandon him in the prison that he created for the both of them. Whether the above paragraph is true or not, there is also an explanation for why the final areas of the game look so much more frightened. It's simple, really. Even though she has been hardened by her bitterness, Mary is still just a little girl. She is rapidly losing her self-control towards the end of the game, after realizing that Garry is trying to manipulate Ib into helping him escape, which would leave her all alone. Her behavior could be said to be typical for a child of her age — she doesn't think things through because she is panicking. Garry is a bad man in her eyes, and she wants to stop him. She may or may not also like Ib as a friend, depending on the player's choices, but if she can't reason with Ib, she'll kill her as well as Gueterna/Garry. Mary behaves so brutally because she feels that she must. She really just wants to go home.
Mary is a real person.Sometime in the past, Mary visited the gallery and became trapped. Like Garry, she had to go through the doll room event and she failed it. However, unlike Garry, there was no one else there to snap her out of it. She lost her mind/memory and eventually became a part of the gallery as a living painting.
Mary is Guerterna's daughter.Guerterna had painted a portrait of her as a tribute to his only daughter.
Almost all of Gueterna's work are people who became trapped in the other world.The game mentions that most of Gueterna's work is not based on real people, excluding works such as the juggler one. The game points out that Mary is not based on a real person, and Garry can become one of the paintings in the gallery in one ending and be forgotten. Who's to say that other paintings aren't like them, if reality can just rewrite itself to conform according to who does and doesn't escape the gallery? It's not as though people who die there will be remembered, and thus none of their paintings are not based on a real person.
The death of a visitor cursed Guertena's exhibitions.One of the books in the gallery mention a girl who suddenly got lost in an art gallery and collapsed. Perhaps this is about an actual incident where the girl ended up dying due to the exhibition staff's negligence. The exhibition became cursed/haunted: just as that girl died, so too will others die like her. Since this troper believes in the above mentioned all-paintings-are-people theory, Mary could have been that girl. Or a later victim of the curse.
Ib is a young Madotsuki.One of the names is not her real one, or perhaps she assumed the name Madotsuki for some reason we're not told (it's not like we're given explanations for anything else). Madotsuki/Ib convinced herself her adventure was a dream (or perhaps it was) as she grew older, or perhaps is involuntarily haunted by strange dreams. Poniko is probably Mary or a parallel to her, and Masada might be a similar counterpart to Garry.
Garry is a hoboThis is an idea I've been nursing for a while, I even posted about it on tumblr a few times. Garry's bedraggled appearance is a big sign; also, it always stuck out to me that Garry ate "a" macaroon and not "some". If you know what macaroons are, like little cookie things, then you'd know that one does not simply eat one macaroon. So I'm thinking someone gave him one? Where I live, free art galleries and the like are common places for the homeless to hang around in during the winter, since they can stand in the heat for a few hours that way.
The Bonus Dungeon is where Guertena keeps artworks that he's either ashamed of or are incredibly personal.Two of the artworks there have had their faces scribbled out, three others are hidden in the darkness out of sight, and a few more are found in a sort of storage area filled with old boxes. As for the highly personal ones, the way to them is blocked by deadly plants that eat you if you get too close; once you get in there, one of them depicts a "tryst", and one of them is in pieces and once you've pieced it together, you learn it is a painting of the man himself. Or at least his hands and shoulder.
Mary needs two people to escapeIn the "Together Forever" ending, Mary's plan works perfectly and she escapes into the real world to live with Ib and her family, while in "Painting's Demise" it is made very clear that something's gone wrong. Both endings have Mary take the place of at least one person which is all that we're told is needed for Mary to get out. So what's the difference? Ib. Without Ib, there's nobody on the other side who even knows that Mary exists. Mary not only needs someone to take her place in the painting world, but also someone else to keep her attached and anchored to the world outside. So long as even one person in the real world knows and accepts her presence there, the rest of the world will follow suit. Without that anchor, she ends up being rejected by both worlds. It's likely that Mary herself doesn't know about that part of the process, which allows for the game's worst ending.
Garry is Guertena after he escaped the gallery world.He sought to invest his feelings into his artwork and did exactly that, and in so doing, trapped himself inside his own creative visions and then had to get out, but just like in the endings where he escapes during the game, he loses his memory. Mary doesn't remember him because he doesn't remember his former self, and it explains how he shows up to a gallery exhibition dressed in such shabby clothes (while not all of them are black-tie affairs, many fine art gallery shows are considered at least semi-formal... but the artist being featured would get much more freedom to skirt the dress code... and he does show up in a jacket). It's also why he's so terrified of the dolls: he didn't make them, so he knows subconsciously that his vision has gotten completely beyond his control.
Guertena's fractured spirit is haunting the gallery world.Putting pieces of his soul into the paintings made him unable to move on after death. Eventually, what remained of him went mad (he could be jealous of the outside world like Mary, or just suffering from the fact that most of his works are based off of bad memories) and began attacking people who entered the haunted gallery. He's the one leaving certain cryptic messages, and in Mary's bad ending, he kills her rather than let his last work escape.