Well she's supposed to be the "villain" of the game but after I read her background I really didn't dislike her, I more grew to like her.
So it went like this. *spoilers*
Mary's trapped in this place, alone, where she never changes and never ages, waiting for her father who will never come back because he's dead. Two people come to this place, Ib and Garry. She became quick friends with Ib and protected her. She was also friends with Garry, but she wanted to get out. In the game, one person has to stay. Its a choice game so it really depends on your choices, but most people get the ending where Mary kills Garry because she went a bit off-the-wall-crazy and after that you kill Mary and escape alone. But I got the ending where Garry dies, but you escape with Mary. I'm sorry if I didn't explain well >.<
Oh and the game is free
Ib