The Killer

"'To rule each prison there must be a warden whose secrecy, caprice, and limitless capacity for violence act as an image of the Fates to be studied and interpreted by those interred by his baleful rule.' —Mappy"The Killer is a mysterious figure who drives most of the events of the series. Very little is known about him other than that he revels in murder, indiscriminately killing potentially thousands of people. His motives are unknown. Many characters in the series seek to exploit the bloodshed caused by him for their own benefit. Despite repeatedly ending up in situations related to him, BB has only once encountered the Killer directly.

Design
The Killer appears to be a man with the head of a vulture, dressed in all black with a red collar, and with a twisted smile. Whenever he is onscreen, the visuals surrounding him change to a stark red and black, contrasting sharply with the generally-colorful nature of the world. As seen in Hands of the Killer, his vision is also tinted deep red.

Prior to Voice of the Killer
Tammy was one of the Killer's earlier victims, being drowned by him sometime in 1992. Dr. Zoo was inspired to found the Murder College after witnessing one of the Killer's murders, though the date and context of this is unknown.

Voice of the Killer
Life Contracts attempted to exploit the Killer's murders to sell people life insurance, even preparing a welcoming party for him in the hopes that they could work together. This backfired, as the Killer is implied to have been the one who burned down the call center.

Hands of the Killer
While attempting to escape from Dr. Zoo, BB opens the protective shutters and inadvertently lets the Killer into the building. Out of sight of BB as she escapes through the vents, he stabs Dr. Zoo to death and slaughters all of the other students of the Murder College, hanging some of them from the ceiling by nooses.

Drool of the Killer
The Killer is the driver of the Tammy bus, as his bird-like face can be seen on the title screen, but only in shadow. He is also implied to be the Donald Duck mascot encountered in the lobby, and his face can be clearly discerned in Tammy's paintings.

Eyes of the Killer
The Killer takes center stage in the finale of the Immersive Theater's production of Melmo the Wanderer as the character "Caesar", a stylized version of himself, as he personally kills the actors playing Mr. Sun and Mr. Moon live on stage. In contrast to his actions with the Murder College, the Killer is shown to be actively working with the Art Appreciators' Club as they bring him sacrifices. This is the only known instance of him cooperating with someone else.

Trivia

 * The Killer's appearance as a vulture, a well-known scavenger, may tie into a broader theme of exploitation of tragedy, as displayed by Life Contracts attempting to profit off of him and the Murder College idolizing him.
 * According to garmentdistrict, the Killer's appearance is based on Max Ernst's "Loplop" series and from Alfred Kubin's "Forgotten, Lost".
 * garmentdistrict originally planned to only have the Killer visible in indeterminate shadow, but discarded that idea early on and went through a host of other Killer designs before settling on the current design.