Phantom Girl

Phantom Girl is one of the Easter Eggs in Yandere Simulator. She can only be seen if the player takes a photo of the bathroom on the third floor. No matter where Yandere-chan is, the Phantom Girl will always be looking at her. YandereDev has stated that she died by a knife stab.

As of the June 16th update, the Phantom Girl is now located in the girls' bathroom on the third floor instead of her former location, which was under the cherry tree behind the school. The former area was just used for testing purposes, since the bathroom wasn't programmed before.

Appearance
Instead of wearing the default female uniform like all of the other girls, Phantom-chan wears a Blue Seifuku, the same as seen in the first intro. Her uniform will not change if customized. In early versions, when the player touched Phantom Girl, the uniform would change to black and red. However, this feature is no longer available in current builds since it was reported countless times as a "bug".

Myth
The Information may be outdated here, since lots of things changed.

A early post on 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."

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

Trivia

 * If you use the Cinematic Mode in the bathroom where she is and take a picture, you'll see her looking to Yandere-chan in normal screen. She is taller than other students due to the fact that she is on her toes.
 * When Info-chan said that the history is repeating itself, in the first 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 original intro, that's probably due to her mother being a Yandere too.
 * Phantom Girl might be a reference to a famous Japanese myth, Hanako-san.
 * According to the myths, Hanako-san is the spirit of a World War II-era girl who haunts school bathrooms. She appears on the third floor and appears when one knocks three times on the third bathroom stall and shouts "Are you there, Hanako-san". However, unlike Hanako-san, Phantom Girl appears when the player takes a photo of the bathroom from the third floor.
 * If you ran past the tree where Phantom Girl used to be located before the June 16th Update, your clothes would change to black and red.
 * The ghost used to be by the tree behind the school for testing purposes.
 * More will be revealed about the ghost in Story Mode.
 * She is planned to be a rival in a DLC (which hasn't been made), and she would want vengeance for what Yandere-chan's mom did to her by getting in the way of her daughter's love life and flirting with Senpai in a human form.
 * However, it should be noted that many plans have changed since the blog that mentions this was written.
 * Her age is probably around 18, because she was murdered in the 3rd floor bathroom, and the 3rd floor's students are around 18.