Madotsuki is the main character in Yume Nikki. Fan speculate that she is young, anywhere from nine to fourteen years old, although arguments can be made for any age.
Not much is known about this girl, and only what can be seen in-game can be considered canon.
She seems to be a hikikomori, or shut-in. This is supported by her refusal to leave her room - instead, she simply shakes her head.