History WMG / Ib

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 [[spoiler:an extremely lifelike yet imaginary girl named "Mary"]].

* [[spoiler:Mary ''does'' consider Guertena her father...]]

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 [[ManEatingPlant 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, [[spoiler:you learn it is a painting of the man himself. Or at least his hands and shoulder.]]

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 [[spoiler:Mary's bad ending, he kills her rather than let his last work escape]].

Garry's characterization as a [[NiceGuy nice]] BigBrotherMentor works fine as a stranger trying to get him and a little girl out of trouble, but if he was supposed to be her brother by blood as well, it would've explained why just him and Ib show up in the Fabricated World -- their parents would have sent him into the gallery with Ib to keep an eye on her, so he would've been present when he and Ib would've been dragged into the Fabricated World together. It would also explain why [[spoiler:Mary becomes Ib's sister if she escapes with her, as Mary is literally taking Garry's place in both reality and in his and Ib's family]], and besides, I wouldn't put it past someone who named their son Garry to also name their daughter Ib. This would put [[spoiler:Ib's DespairEventHorizon if he goes insane]] in an even sadder light, and would mean their family goes through heartbreak and has an unhappy ending in almost ''every'' ending, [[spoiler:whether [[LaserGuidedAmnesia they know it or not]]]].

The reason he was changed to a stranger in the finished game is probably to set up a BittersweetEnding and a GoldenEnding so there is more replay value. If they're siblings, [[spoiler: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 [[spoiler:they don't remember their harrowing experiences]]. But if we make them strangers who'd only met in the Fabricated World, [[spoiler: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 GoldenEnding 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 [[spoiler: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'' [[spoiler:Together Forever becomes more sinister, as Mary successfully escapes into the real world by killing Garry and taking his place as Ib's sibling]].