Phantoms

Phantoms are a type of undead. In common understanding, phantoms are either synonymous with ghosts or a specific kind of ghost. In Monster High, they are still undead and elusive, but solid beings.

Etymology
"Phantom" descends from the Ancient Greek word "φάντασμα" ("phántasma"), which can be translated as "apparition", "vision", "dream", and "fantasy". It comes from the verb "φαίνειν" ("phaínein"), which meaning comes down to "to (visually) make the presence known of".

The word appeared in the English language around 1300 as synonym for "apparition", "vision", "dream", and "fantasy". It became a synonym for "ghost" one to two centuries later. The additional meaning of "something having the form, but not the substance, of a real thing" is from the early 18th century, following from the same mindset change that also affected the meaning of "simulacrum".

Fiction
The Phantom of the Opera is a novel written by Gaston Leroux and originally published as a serialisation from 1909 to 1910. The story is set in Paris, France, but the year is not noted. Most adaptions of the novel specify it to take place in the 1870s or 1880s, depending on the adaption, because the main location of the story, the Palais Garnier, was built from 1861 to 1875 and the titular character had at least a few years of history with the building prior to the events of the novel.

The Phantom, who also goes by the name Erik, though this is not the name his parents gave him, is born in a small town outside of Rouen, France. Having the misfortune of being born with a severe deformity that gives him a corpse-like appearance from head to toe, he flees his home at a young age because not even his own mother could stand to be near him. He joins a group of Romani and earns his stay as an attraction in freak shows, where he is known as "The Living Death". Though far from ideal, it is a home through which he learns the skills to become an illusionist, magician, and ventriloquist. It is also during these years that he discovers that he is in the possession of an unearthly singing voice. For all this, he is invited to the palace of the Shah of Persia at some point in his adult years. He proves himself a capable architect, but when his life is threatened, he moves on, traveling across the globe until he returns to France to lead a more peaceful life. He settles in Paris and becomes part of the Palais Garnier construction team. This position allows him to turn the building into his own personal hideout, with many doors and passageways only he knows about. He builds himself a living space in the lake below the Palais Garnier and from then on "haunts" the building, thus earning the title of Phantom of the Opera

One day, a new singer joins the Paris Opera: Christine Daaé. The Phantom falls in love with her and tries to acquire her for himself. The first time he abducts her to the cellars of the Palais Garnier, his plan is to only keep her for a few days in the hope she will come to love him. However, she unexpectedly takes off his mask and is horrified by his face. Afraid she will leave, the Phantom decides to hold her captive forever, but still agrees to release her after a couple of weeks if she promises to return. Instead of doing that, Christine meets up with her mutual love, Raoul de Chagny, and agrees to run off with him, but not before having sung a final song for the Phantom. The Phantom, however, has spied on Christine and wants revenge for his betrayed trust. He abducts her a second time the following night and tries to force a marriage on her, holding everyone in the building hostage with explosives planted in the cellars. Christine still refuses, so the Phantom reveals that he has captured Raoul (as well as a former friend of his, the Persian, who had agreed to help Raoul) and will murder him if Christine still rejects his marriage proposal. To save Raoul, Christine agrees to be the Phantom's bride and allows him to kiss her. This moves the Phantom greatly because not even his own mother ever allowed him a kiss. For this gesture, as well as a kiss from her, he decides to let Christine go and be with Raoul, on the condition she'll return to bury him when he dies, which occurs only a few weeks later.

As an immensely popular story, the novel The Phantom of the Opera has received many stage and film adaptations of which a few even outshine it. The most notable of these are the 1925 Universal Pictures The Phantom of the Opera movie, starring Lon Chaney as the Phantom, and the 1986 musical The Phantom of the Opera'' Andrew Lloyd Webber. The 1925 movie's fame comes from the fact it was the very first adaption of the novel, and of all adaptions it follows the novel the most faithfully. The 1986 musical is best known for the liberties it takes with the Phantom's appearance, enhancing the mystery and appeal of it by only making half of his face disfigured. This change was not done for this purpose though. Initially, the Phantom would have the full deformation of the novel, but then it was found that such an appearance would make expression onstage difficult. As a solution, the Phantom's deformity was reduced to half his face so that he would only need to wear half a mask. The deformity in the musical includes a gash on the right side of his partly balding head with exposed skull tissue, an elongated right nostril, a missing right eyebrow, swollen lips, different colored eyes, and a wrinkled right cheek.

Monster High
The Monster High phantoms are Operetta and her parents as well as Dr. Boolittle. Mr. Rotter is not confirmed to be a phantom, but since his design is based on Lon Chaney's Phantom of the Opera, he is logically one too. Based on these five, phantoms can have purple skin, green skin, and blue skin. Invisi Billy notes in his New Scaremester diary that he is regularly mistaken for a phantom, suggesting they can have white skin too.

Operetta was one of the four characters originally designed for Monster High, but during the three-year development period her role was given to the mummy Cleo de Nile. Operetta was given fictional presence in Holt's Basic diary, but didn't make a personal appearance until late in Volume 2 of the cartoon and wasn't fully incorporated until the Campus Stroll line. She and Howleen Wolf, if perhaps in name only, had to wait the longest of the original four to be included. In Operetta's case, this presumably was done because the Phantom of the Opera is not necessarily a monster in the source material and Operetta would fit better as a building stone of the franchise than a foundation pillar.

Prior to Operetta, the cartoon introduced Mr. Rotter. He is notably not one of the original teachers as introduced in the Fearbook, yet has grown to be one of the three most prominent members of the Monster High staff in fiction. As aforementioned, he is designed after the Phantom of the Opera as the character appeared in the 1925 movie The Phantom of the Opera. In Ghouls Rule, he even displayed a preference for using secret passageways like the Phantom of the Opera is known for. He is not the Phantom himself though, who is a separate character but coincidentally also a (night) teacher at Monster High.

Because phantoms in Monster High do not match the ghost-like qualities of the source creatures, they are among the most ill-defined species in the franchise. Both Operetta and Rotter prefer to control their own coming and going, but voice powers seem to be exclusive to Operetta and potentially her family. Little is known about Dr. Boolittle but for the fact he has no problem spending decades away from society for the purpose of research. As such, reclusiveness appears a defining characteristic, which would be based on the behavior of the Phantom of the Opera in The Phantom of the Opera.