This is an idea I've been kicking around for quite some time. Even though I am a huge fan of Ib and Garry's ending, I sympathize with Mary. Here's a little tribute to her. (This is not an attack on Garry. I love Garry, actually~)
Disclaimer: I don't own Ib.
Garry liked to play favorites; Mary knew that. She could tell by the way he looked at Ib: the way his eyes would widen with oh-so-sincere concern when she would get hurt, the way he smiled at her as he held her hand when they both walked down the long dark halls of the art gallery. He treated Mary with contempt and didn't even as much as look at her. He didn't have the same concern in his eyes when Mary got hurt and he certainly didn't hold her hand nearly as tenderly as he held Ibs' hand. Ib was his favorite.
This upset Mary because she liked them both, even though Mary technically liked Garry first. She kept a close eye on them once she had trapped them in her fabricated world. Garry was quick to help Ib, which was good; Mary was worried about someone as small as Ib walking alone in her world. Her world was dangerous; Mary knew that better than anybody. He was kind, Mary noted when Garry held out his hand to Ib for the first time. Ib was quiet, but that must mean she's a good listener, right? After a while, Mary decided to join their little group to see if they would like her as much as she hoped they would.
But when she joined their group, it was apparent that Garry only cared about Ib. She didn't have to ask Garry who he'd like to go with the other world the most, and who he'd rather leave behind. She asked Ib though, and Ib said she would sacrifice herself. She was kind too, Mary noted then. Ib was, as Mary had suspected before, a good listener. And above all, she liked Mary. She even told Mary that her doll was cute and agreed with her that those bunny sculptures were cute too. They must have a lot in common. They could even become friends. Best friends.
Then, Mary didn't care about Garry anymore. She had Ib. If he was going to play favorites, then Mary would steal his favorite.