Petunia is the star of the game Petunia the Yellow Mouse. She is, as the game's name implies, a yellow mouse, and only one of two known in the world - the other one being her brother, Charlie.
Petunia is a baby, and has no concept of reality. Strangely, her perception is actually enforced on reality itself, and as such, she is capable of warping the world around her, even if she's unaware of what's going on.