She is an evil entity that appeared in multiple characters dreams, some of them felt sick after waking up, the first person that mentioned this dream was Ai.
It was later revealed that Kumiko was a devil, and was infiltrating dreams as this person.
As described multiple times, it looks like a thin woman in a red dress, with long brown hair.
She has only been seen in dreams, and says: "Come over here".
- not being real, she never had her name revealed, but was mostly refferec to as "the woman in a dream" or more commonly "Red one-piece", reffering to her clothes.