I'm not sure specifically as there could be many different 'ib game stories'. It might be about a character named Ib in a particular game world, perhaps her adventures, challenges she faces, or relationships she has with other in - game characters.
I'm not certain, but if Ib is the main focus, then she's definitely one of the main characters. There may be other characters like the people she meets in the game world, perhaps some that help her or some that are antagonists.
One popular theme is the exploration of the relationships between the characters in the game. For example, how Ib and Garry interact and form a bond during their strange adventure in the haunted art gallery. Another theme could be the backstories of the various creatures and paintings in the game. Some fanfictions might also focus on alternative endings, like what if Ib made different choices at certain key points in the game.
The distorted art is really scary. In ib horror game stories, paintings that should be normal become grotesque, with faces melting or bodies in unnatural poses. It's very disturbing.