Phantom Girl

The "Phantom Girl" is an Easter Egg in Yandere Simulator. She can be seen if the player takes a photo of the bathroom in the third floor. No matter where Yandere-chan is, the Phantom Girl will always be looking at her.

As of the June 16th update, the Phantom Girl is now located in the girls bathroom on the third floor.

Appearance
Instead of using the default uniform like all the other girls, she wears a Blue Seifuku, the same as seen in the outdated intro. In early versions, when the player touched Phantom Girl, the uniform would change color to black and red. However, the feature is no longer available in current builds.

Myth
Information may be outdated here, since a lot of things changed.

A early post in the Yandere Simulator Blog, named "Halloween and DLC" featured some ideas for the game, which included the Phantom Girl. In this post, the ghost had a description:

"Yandere-chan’s mother was also a yandere girl in high school, and killed a female student in one of the school’s bathrooms. The player can actually see the ghost of the girl that Yandere-chan’s mother killed by walking into the bathroom and taking a picture. But what if that ghost had a chance to take revenge on Yandere-chan’s mother, by thwarting her daughter? The ghost might decide to channel all of her energy into gaining a physical form, and romancing Senpai-kun before Yandere-chan can confess to him...Yandere-chan would not be able to “kill” the ghost, and would have to expel her using unconventional methods...Perhaps the occult club would hold the key to getting rid of her."

In the early versions the Phantom Girl stood under the cherry tree, now she can be found in the 3rd floors bathroom.

It's also a myth that under the cherry tree where the Phantom Girl was, if a student confess themself to another student, their feelings would be accepted.

Trivia

 * When Info-chan said that the history is repeating itself, in the outdated intro, she was referring to Yandere-chan's mother's history.
 * Also, when Info-chan said that killing students is in Yandere-chan's blood in the outdated intro, that's probably due to her mother being a yandere too.