Humans are a type of non-monster able to have lacks of powers or special abilities, and are also creatures with two arms, two hands, two legs, two feet, and able to have different skin colors from Caucasian to Hispanic to black to Indian (from India), to Native-American. They are not monsters, clearly, as they are humans, but some able to live beyond the boundary such as a superhumans. Like some might have supernatural powers, some are plain and regular, as "simple normies" so to speak.
They can even fuse with some kinds or classes of monsters, such as to create dhampirs, which are combinations of humans and vampires. Humans can be very subtle creatures, yet some destructive and wanting to cause harm to monsters. Most of them have no special abilities, like monsters have.
History of Humans
Humans are also known as homo sapiens. A homo sapien is a complex, but good natured and friendly animal. Some are and some aren't. Some humans are not, but they are also part of the Hominina clade, a type of "homo" clade, which homo is a type of genus which includes modern humans. Humans, or normies started out as cavemen, then hundreds of years later, went into the Renaissance period. History of them goes as far as the history of monsters.
Humans in Monster High
Most known about ones are Jackson Jekyll, his father Dr. Jekyll (grandfather in the Lisi Harrison books), Sydney Jekyll, who is his mother in the books, Van Hellscream, Lilith Van Hellscream, Chad and Clair. The main one in the Monster High franchise is Jackson Jekyll. Jackson is brilliant, nerdy, and also the alter-ego of Holt Hyde. He's a bit geeky and dorky, but is cute and sensible. He is voiced by Cindy Robinson, the same voice actress of Operetta, who is a phantom, and Holt, who is a fire elemental.