Nanashi, as she appears in the Needle World.
Nanashi - Japanese word for No Name
Unknown, though she is taller than Usotsuki
On your first visit to her area, she appears normal. However, hitting her multiple times will reduce her first to bleeding from the right eye, left arm, neck, and stomach.
Continuing to hit her reduces Nanashi to a crawling, legless, bloody heap. Hitting her further will stop her crawling -- she is presumably, at this stage, dead. She must be killed in order for Usotsuki to leave the Needle World.
It should be noted that she is one of the only characters in Yume Nikki and its fangames to die in this more realistic manner; another significant example of this would be Sabitsuki, from .flow, in the third ending.