All monsters are combinations of human and non-human features. There is immense variation in the ratio of human to nonhuman features: they range from mostly human (e.g. elves, which appear human except for their pointed ears) to mostly non-human (e.g. Large Spider, who resembles a spider with a human head).
Pure monsters are all female. Consequently, they require males of other species in order to reproduce. Human men are by far the most commonly used for this purpose. Some monsters (such as Queen Ant) are capable of reproducing using nonhuman males, but this results in lower-quality offspring. There are rare cases of a monster giving birth to a male human with monster blood. The royal line of Sabasa arose from the union of the first Sabasa King and the ancient monster Sphinx. It is also apparently possible to transform a human man into a monster, according to Sphinx, but this has not been further explained.
There are numerous races of monsters. In alphabetical order, these are: animals, chimeras, demi-humans, dolls, dragons, elves, fairies, ghosts, harpies, insects, kitsunes, lamias, land-dwellers, mermaids, plants, scyllas, sea-dwellers, slimes, succubi, vampires, yoma and zombies.
Monsters usually have a leader, a Queen of some sorts of their species, but all monsters are ruled over by the Monster Lord, who exerts power to maintain order of the entire monster race. However, monsters often tend to do their own thing and are very individual by nature, and they easily twist the commands of their lord into their own ideals, or even rebel against the Monster Lord.
Monsters are often described as "sinners" and almost all of them care little about anyone but themselves. They are infamous in the entire world due to their lustful nature, raping men for their own sexual pleasure, enjoyment, food, or even reproduction, and will thus almost always attack men first. Their personalities differ, although many are very sadistic and/or carefree.
Relationships with other races
Monsters are dependent on human males to reproduce. However, the faith in Ilias means that most humans have been hostile to monsters for most of their history. Ilias' first commandment forbids humans from having intercourse with monsters.
In the original Monster Girl Quest, this belief was reinforced by the example of Remina: once a city where humans and monsters lived together, Ilias had her forces slaughter the inhabitants, and humans came to believe that monsters were responsible. As a result, most humans fear and hate monsters, making it difficult for monsters to reproduce. For their part, many monsters attacked humans, raping and sometimes killing them. However, there were still some cases of monster-human coexistence. For instance, Iliasburg accepted the Four Bandits, the humans and harpies of Happiness Village eventually learned to get along (with the humans providing mates and the harpies helping with labour), humans and mermaids live together in Port Natalia, the Sabasa royalty takes great pride in their monster bloodline, humans and various species of monsters live together in peace and harmony within Grand Noah, the humans of Yamatai worship monsters instead of Ilias, and the humans of Grangold agreed to coexist with the Ant Girls. After Ilias' fall, humans and monsters throughout the world begin to work towards coexistence.
In Monster Girl Quest: Paradox, the Slaughter of Remina never occurred. Combined with the weakened faith in Ilias, this resulted in monsters and humans generally coexisting. This is best illustrated by monsters living even in places such as Iliasville and San Ilia. However, there are still some areas where hostility to monsters remains, such as Luddite Village.
As the creations of opposing goddesses, monsters and angels have historically been at odds with each other. During the Great Monster Wars, the two races and their creators fought with each other. During this time, monsters could harm angels and they were able to get the upper hand, though Ilias was able to force a stalemate by effectively taking the world hostage. After this period, monsters were unable to harm angels due to the latter's immunity to most attacks (it is still unknown why monsters could harm them before).
In the original Monster Girl Quest, monsters and angels continued to be hostile to each other. Angels slaughtered the population of Remina, causing humans to blame monsters for the slaughter. However, the monsters were unaware of the angels' involvement, as shown by Alice's surprise at finding an angel feather in Regina's ruins. Later, the angels attempted to wipe out monsters and humans alike. After Ilias' fall, the angels now live in peace with monsters and humans.
In Monster Girl Quest: Paradox, the relationship between monsters and angels appears to have softened. In the village of Enrika, angels live alongside elves. When Luka's party reach the Snow Continent, many of the angels there are friendly towards the monsters in said party.
Monsters were born out of creatures and objects that were genetically mutated by the dark genes of Alice I. However, Goddess Ilias, the manifestation of light, grew jealous of Alice I because the mutation allowed her to create creatures at a much more faster and controlled rate.
One thousand years ago, Ilias started the Great Monster Wars, a historical war between the monsters and angels. While the monster force was said to be superior, the goddesses eventually realized their constant clashing would ultimately destroy everything they created. However, Ilias continued her onslaught and forced Alice I to surrender, on the condition that Ilias would not attack monsters further. However, Alice I included a counterbalance in the seal: if Ilias were to break her promise and kill a large number monsters, the seal would break and Alice I would be freed. Ilias, however, worked to find a way around this. To keep the darkness of the world in check, she recruited to her side Black Alice (a descendant of Alice I with intense hatred for humanity) and had Promestein create the Chimera monsters.
Monster Girl Quest
As the years passed, monsters and humans eventually came to coexist. Ilias could not stand for that and successfully drove a wedge between the two by destroying Remina, one of the most famous coexisting cities, thus causing the two races to blame each other for the city's destruction. In reality, Ilias appeared to her religious humans and ordered them to kill monsters and the Monster Lord, falsely claiming that they are her "failed creations." Heroes were deployed to combat the monsters, however there are very few humans who can actually defeat any monster on even terms; this is supported by Granberia's Monsterpedia Entries.
Less than thirty years ago, the then Monster Lord Alice XV wished for coexistence between monsters and humans. She allowed herself to be killed by the hero Marcellus, as he and his allies had similar intentions. However, before they could carry on her wish, the young Alice XVI entered the room and angrily attacked them for killing her mother. As a result, the rift between monsters and humans continued.
However, Alice XVI later encountered a young man named Luka who, by some strange twist of fate, was the son of Marcellus. They travelled around the world together, resolving conflicts between humans and monsters. Luka was tasked by Ilias herself to defeat the Monster Lord, Alice XVI, but after seeing the world with her he could not bring himself to it and turned against Ilias. The evil goddess decided to take matters into her own hands and sent her angelic army, coupled with Black Alice's rebel monsters and Promestein's chimeras, to assault the entire world and eliminate the coexisting humans and monsters. However, the monsters and humans joined forces and fought back.
Moreover, Black Alice and Promestein betray Ilias in their plan to become the ultimate entity of light and darkness by incorporating themselves with the power of angels and monsters; Black Alice absorbs Ilias into her body while Promestein injects herself with the drug White Rabbit that contains Alice I's genes. Luka fights and defeats the two, but Ilias emerges from Black Alice's remains and reveals her true master plan was to become the ultimate entity and assimilate the power of monsters and Alice I into her body. The humans and monsters fighting all over the world against Ilias's forces pray for Luka and Alice XVI's victory, and with their combined powers they put an end to Ilias once and for all. Finally, there is true peace between humans and monsters.
Monster Girl Quest: Paradox
As the Great Disaster happened instead of the slaughter of Remina, monsters and humans banded together to face this challenge, and, at the same time, the faith in Ilias declined as not a single prayer had an answer. As a result, both species are way closer than in the original world, with monsters being openly welcome in Iliasville and even secretly accepted in Ilias Temple as long as they do their own work and control their pulsions. However, as in the original universe, a lot of monsters prefer roaming the world to get their way with any humans unlucky enough to cross their path. Most of them can be recruited by Luka as allies, and will fulfill any of his more personnal request if they take a strong enough liking to him (or may still kill him instead. They're still quite fickle.)
They're supposingly led by Alice the Sixteenth, but she was forced to leave the throne unattended after being sealed by the White Rabbit, and Alice XV took it back after being supposedly dead for attempting to explore Tartarus. Simultaneously, a revived Black Alice and another mysterious Alipheese the 17th also lay claims of legitimacy to the throne, further muddying monster's politics.
Furthermore, the Lilith Sisters, a trio of powerful succubi, is secretly wrecking havoc in the world with the goal to get as many people killed as possible for the sake of their own plan supported by the Fifteenth. With the help of the Tamamo No Mae of another world, they ignited a war between Grangold and Grand Noah. When said war is stopped by the actions of Luka and several of his companions, Alice XV appears as a projection at a banquet and declares a war of extermination upon humankind. Each city is attacked by a specific race that resented humans for a reason or another. Luckily, Luka manages to push them back and managed to beat each queen leading the assault, more or less bringing the war to a stop.
In the aftermath of the war, a lot of humans came to resent monsters, worsening the relations between both species as humans no longer feel safe in the presence of monsters, even for the more civilized ones.
Not much is known, except that the word of the strong is the law, and that the Six Ancestors are at the top. They're free to do as they please, but their actual day-to-day situation is unknown so far. The Lilith Sisters more or less come from this world. To be exact, their souls has been sent to fuse with their body present in the Paradox World. The Tamamo No Mae responsible for manipulating Grangold also comes from there. At the end of Alice's route in part 2 the three sisters ask for Luka's support in their plan to save as many souls in this world as they can.
Monsters have been mostly exterminated by the Goddess Ilias. A small number remains alive in the region of Noah, under the management of Uriela, and are treated as second class citizens.