Phantom Girl

Phantom Girl is one of the first Easter Eggs in Yandere Simulator. She can only be seen if the player takes a photo in the girls' bathroom on the third floor. In the November 15th, 2015 Build she is on the girls' bathroom on the right side of the school. 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 from Ryoba Aishi around April of 1989.

As of the June 16th, 2015 Update, the Phantom Girl is now located in the girls' bathroom on the third floor instead of her former location, which was under the sakura tree behind the school. The former area was just used for testing purposes since the bathroom wasn't programmed before.

There may be an event where a member of the Occult Club can interact with the Phantom Girl.

To not cause a time paradox, the Phantom Girl will be killed in the intro animation for 1980s Mode.

Appearance
The Phantom Girl has transparent grey skin and no pupils. She also has the same hairstyle as Yandere-chan. 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".

Background
The Mysterious Tapes give a hint to her backstory, given that Ryoba Aishi was the second yandere and the Phantom Girl's death instigated The Journalist meeting the yandere.

The Phantom Girl died around April of 1989. The Journalist says that there was a murder at a local high school in Mysterious Tape #1. Mysterious Tape #3 shows that "the girl who was murdered at the beginning of the school year had a crush on the same boy that was being stalked," which is why Yandere-chan's mother killed the Phantom Girl.

The Phantom Girl has haunted the third floor girls' bathroom for a few decades now, probably because she is too full of hatred to depart for the afterlife.

Her age is probably around 18, because she was murdered in the third floor bathroom, and the third floor's students in Japan are usually around 18.

Myth
Some of the information here may be outdated, since lots of things have changed.

An early blog post on YandereDev's WordPress, named "Halloween and DLC" featured some ideas for the game, which included the Phantom Girl.

As it was told, Yandere-chan's mother was 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 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.

Trivia

 * If Yandere-chan uses the Cinematic Camera Mode in the bathroom where she is and takes a picture, the player will see the Phantom Girl looking to Yandere-chan in normal screen.
 * Phantom Girl was not a reference to Hanako-san. The Phantom Girl was suggested by an anonymous fan of the game. YandereDev doesn't know what inspired that person's idea. The Dev also states that in the first month of development he wanted to add ghosts which means that he must have had the desire but the anonymous fan gave the substance to the making of the Phantom Girl.
 * A ghost haunting a bathroom is also a common myth in Japanese schools.
 * If Yandere-chan ran past the tree where Phantom Girl used to be located before the June 16th, 2015 Update, her clothes would change to black and red.
 * More will be revealed about the ghost in Story Mode.