Various species exist in the Kuroshitsuji universe. Below is a brief explanation of them.
- Human: The humans of Kuroshitsuji are generally residents of England during the Victorian era. They are incapable of possessing supernatural abilities at birth. However, they can form Faustian Contracts with demons, wherein the latter fulfill their wishes, and in return, the human sacrifices their souls for consumption.
- Furthermore, there are two different species that are born after a human has passed away. These species include:
- Grim Reaper: Grim Reapers are supernatural beings who review lists of humans about to die. They are to remain neutral between affairs that have no relation to them. It is to their discretion whether or not an individual is allowed to continue living, based on the content of his or her Cinematic Record. If the Grim Reaper does not notice any reason to allow the person to live (some kind of history-changing quality), his or her soul is consequently collected. Each Grim Reaper wields a Death Scythe, which serves a dual-purpose of being a weapon in combat and reviewing and collecting souls.
- Bizarre Doll: This race appears solely in the manga. Bizarre Dolls are soulless, carnivorous beings who prey on humans. It was initially believed that they are erroneous experiments of the Aurora Society's medicine. In truth, their creator is Undertaker, who has attached fake continuations to the Cinematic Records of dead humans. This causes them to attack and rip open living humans in search of a soul, having lost theirs when they initially died.
- Demon: Demons are supernatural beings who occasionally form Faustian Contracts with humans, offering a range of services in return for the future consumption of the individual's soul. Pushing aside emotions, they strictly follow their two aesthetics: one, their masters' orders are absolute, and two, obey those orders while keeping their contracts.
- Ghost: This species appears solely in the anime. Ghosts are the result of humans who have died, but for some reason, cannot move on from this world. They remain attached to the human world due to their variety of human emotions. The existence of this race is contradictory, since the Grim Reapers collect all humans' souls at the time of their deaths, and a ghost remaining in the human world should therefore be impossible.
- Doll: This species appears solely in the anime. Dolls are created from humans and other materials, including wax, silver, gold, steel, iron and stone, and are stuffed with materials like sawdust, sand and straw. They are animated and can be manipulated like puppets. While they appear nearly identical to their original human forms, a distinguishing difference is that their movements are robotic and unnatural.
- Fallen Angel: This species appears solely in the anime. Fallen Angels are supernatural beings whose main goal is to purify 'unclean' humans from their sins. They possess the abilities to fly and transform to their human form with ease. Furthermore, they also have the special ability to alter a person's past by modifying their Cinematic Records.
- Demon Hound: This species appears solely in the anime. Demon hounds are a special type of dog with supernatural abilities. Furthermore, they have the ability to transform from a dog to a human and vice versa, but retain their dog-like disposition at all times.