As a WMG subpage, all spoilers are unmarked as per policy. You Have Been Warned.
While people seem to think Garry is a young adult, most of the evidence given indicates that he's likely a minor, probably 16 at most.
- In the gameplay there are sections of text on a wall, presumably written by Mary, inviting Ib to be with her in a world with no adults. Now, at first glance that may seem to foreshadow what will likely happen to Garry — Mary seems to hate adults in general, and Garry being one would explain why she tries to get rid of him (and succeeds in most cases) — but since she herself brought Ib and Garry into the twisted gallery world, it would make no sense for Mary to bring in an adult. She must want to get rid of Garry not because of his age, but because he's a rival for Ib's attention and affections.
- Though, to be fair, Mary does ask Ib if Garry is her father — but since she doesn't seem to know much about real humans in general, she may well be ignorant and just THINK Garry's an adult, or perhaps teens and adults are basically the same in her eyes.
- Where does it say that Mary made the writing? Writing can appear after she's dead in the Bonus Dungeon, and we know that one doll was writing for Garry.
- Garry also doesn't quite seem mature enough to be a grown-up. He's certainly more mature than the children, but his relationship with Ib isn't really fatherly as much as it is brotherly.
- If he's 16 or so, he wouldn't be an adult, but he would be able to get to an art gallery on his own; he wouldn't be old enough to drive (by Japanese standards), but if he lived close enough he could easily have walked there himself.
- Actually, Japan's policies might not matter at all. None of the characters in the game have Japanese names, and it's more likely that the game takes place in a different country with a different driving age altogether.
- They're in at least the 63rd century. Who says humans even drive anymore?
- Dyeing one's hair purple seems more like something a high school student would do than an adult.
- And since he does seem to care about fashion and aesthetics to a degree, maybe he's just a high school art student (perhaps in I.B. art classes; this troper takes one and we're required to visit an art gallery a certain number of times per term. Plus, him taking IB classes and being in the game Ib would just be hilarious).
- There's a moment where Mary ask Ib if Garry is her father so In-Universe he do looks like an adult.
- Or alternatively those pictures of Ib and her family are just her hallucinations, evidenced by Garry's initial confusion when Ib panics at "Couple".
The remnants of Guertena may live on as the spirit of the gallery, and when Ib entered, he was immediately drawn to her, part of him fond of her as the embodiment of the woman he once loved as he had known her and so wanted to bring her near, while the other part of him hated her as the woman who left him and so wanted to punish her. Thus he drew her into his world. Garry was drawn in possibly because he resembled a younger version of Ib’s dad, being the soft and sensitive sort of man that had won Ib’s mom’s affections. The gallery seemed quite keen to torture and punish him, and this may explain that as the gallery is the “World of Guertena.”
Bring Mary into the mix as the embodiment of Guertena’s loneliness, curiosity, and bitterness and you have a repetition of the dynamic that had played out previously in the real world now playing again in the remnants of Guertena’s soul. Mary, like Guertena, wants to love and be close to Ib even though Ib doesn’t necessarily feel the same way, and she bears an irrational hatred of Garry as a rival for Ib’s affection, wishing she could just kill him. In her mind, if she could just do that, then she could be happy; she could have Ib and the life she always wanted, mirroring Guertena’s wish to have Ib’s mom and the life he always desired. It’s possible that this is why the gallery lets Mary leave when she does so with Ib: it’s not about exchanges, but instead about Guertena being happy that Mary was able to do what he was not and so warping reality to give Mary her desired life as Ib’s sister.
As a bit of additional conjecture, it’s possible that Guertena envisioned Mary as the child he never had, the child he could have had with Ib’s mom, a symbol of the happiness and love he was denied. Thus Mary was bound up and effectively animated by these very emotions. Indeed, she’s the only painting that seems capable of continuing Guertena’s work, being able to create or at least heavily manipulate the sketchbook world. And had Guertena ended up with Ib’s mom, then Mary would have been Ib’s sister. In that sense, Mary and Ib becoming sisters really is Guertena’s happy ending, at the minor cost of Garry’s soul.
We know that Guertena did have children, but it is possible that one of them died before birth. Art is a very common tool people use to vent their mourning. Mary is not based on a real person, but she might be Guertena's theory of what his daughter would have looked like had she not died. Her dress looks like a school uniform (she's wearing loafers which are common school uniform shoes) which hints that the Guertenas already had a school picked out for their daughter.
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 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 does consider Guertena her father...
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.
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.
- Madotsuki actually means "window" in one interpretation, whereas Ib is basically the same as "Eve" (again, in one interpretation). It's likely Madotsuki is a title she's given in Yume Nikki since there's no dialog to have anyone address her as such, the same as her name being PC. So, this WMG could totally work.
- The main problems of this is that Garry's clothing is how some people wear stuff like that (torn jeans and the like). It also seems that Garry uses his lighter a lot, which could mean he's a smoker, which isn't a cheap hobby (unless he uses it to light fires in general). Finally, Garry seems educated and coherent (many hobos are mentally ill or have some form of disability). His feminine appearance makes him look well-kept (or clean at least) and his hair seems to be dyed, which is the kind of thing a hobo wouldn't bother or couldn't afford to pay for.
- He also says at one point that he considered wearing nicer clothes to the museum.
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.
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.
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.
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.
As is hinted at in the Forgotten Portrait Ending, dying in the gallery world seems to make a painting of you appear in the real world gallery. Now, in the bad ending where Mary decides to stay so she can be together forever with Ib and Garry, she holds a welcoming party for the both of them... which seemingly involves giving their roses to the Ladies in Red/Blue, thereby killing them. This seems to be more than pure insanity. What if she wants to make them into fellow paintings?
There are a few hints for that:
- The fake!Garry you meet could simply be painting!Garry come to make Ib stay in the Gallery. Granted, it could be another illusion like Ib's mother. But unlike with Ib's mother, where we clearly get to see Ib following nothing, the same cannot be said for the Garry apparation.
- In the Together Forever ending, we do not get to see Garry's painting even though he died in the Gallery, but in this case Mary took his place in the real world, while Garry took hers in the Gallery. Mary's painting is not a part of the exhibition, so this could mean that the Forgotten Portrait is wherever hers was kept.
- The whole party is meant to be a Welcoming Party for Garry and Ib, aka the moment where they permanently become a part of Guertena's Art Gallery.
As good a guess as any.
- By that logic, a 90-year-old would have 45 petals. I can't envision a 90-year-old shrugging off the sort of beating it would take to lose 10 petals, let alone 40.
- I agree with the above comment. The petals likely don't correspond to age at all. Maybe they're related to body size; Gary could pretty easily be twice as big as Ib (though the sprites make it a bit hard to tell).
The reason he was changed to a stranger in the finished game is probably to set up a Bittersweet Ending and a Golden Ending so there is more replay value. If they're siblings, just Ib and Garry escaping together would be a happy ending, and them not remembering the Fabricated World could only be a bonus from their own point of view; a player could very easily get that ending on the first try without too much effort, and be satisfied with it without feeling too cheated that they don't remember their harrowing experiences. But if we make them strangers who'd only met in the Fabricated World, Ib and Garry wouldn't remember each other in reality, which means we get two endings out of it — the sadness of them not remembering each other and parting as strangers in "Memory's Crannies", and the Golden Ending of them remembering and the implication that they'll stay friends afterward in "Promise of Reunion"; a player could still get the former on their first try without too much effort, but will probably be upset, have the incentive to keep trying for a better ending, and be rewarded with the latter, so it's a richer experience for the player. Plus, them having to remember to get the best ending means the player ends the story satisfied and on the same note as Ib and Garry.
- The counterpoint of having Garry as her actual brother is that the other ending Together Forever becomes more sinister, as Mary successfully escapes into the real world by killing Garry and taking his place as Ib's sibling.
If I had children, Garry would be at the top of the list of video game characters I'd trust to babysit them.