Sara is the girlfriend of Furuya. She is considered the most beautiful girl of the school.
Spoiler, click to view
She later became a spirit to "hang out" with Furuya, while she was actually haunting him. Spoiler, click to view
It was later revealed that she wanted to curse Furuya, so she acted like a normal girl to make him guilty enough to commit suicide, all so she could touch and control him again.