Grimm, or the Creatures of Grimm, are the universal antagonists of Remnant. They are wholly driven by destruction; they hold no enmity towards normal animals, but will attack humans and faunus on sight. They lack a soul and the ability to use aura. Grimm are drawn to negativity, and often congregate around the sources of emotions such as fear, envy, sadness, and hatred. The Grimm are said to have appeared soon after mankind, and they’ve targeted humans and faunus for all of remembered history. They nearly wiped out humanity, until humans discovered Dust, allowing them to fight back and defend themselves from the Grimm. Humans then established their kingdoms, which were able to survive and prosper.
Grimm appear to be the predominant species of Remnant, with humans and faunus pushed primarily into four pocket settlements. Villages outside the kingdoms have mixed success and failure, and attempts by the kingdoms to expand are often met with resistance and even fail and become overrun with Grimm.
The origins of Grimm are unclear; most of humanity is completely unaware of how they form. Ancient human cultures believed that Grimm were animals possessed by evil spirits, however, this theory has been disproven over time. With the discovery of new types of Grimm every day, scientists have far more questions than answers.
There are a variety of types of Grimm which come in many shapes and sizes; their size appears to be determined by age. Their shape is often based off different animals, with ursa looking like bears, nevermore looking like birds, and beowolves looking like bipedal wolves. They are said to be the only creatures without souls, and thus without aura, but they make up for this with strength, durability, and savagery. They are attracted to negativity and hostility, and will even join in on attacks in progress between humans if the humans being attacked begin to panic. Grimm typically form packs, and while some may wander alone, they will inevitably rejoin a group to continue their instinctive drive to hunt down the people of Remnant and anything they have created.
The longer a Grimm lives, the larger it becomes, and older Grimm are able to learn from their experiences. They are initially mindless, however, once a Grimm has lived for hundreds of years and grown enormous, it learns not to take unnecessary risks. Instead of attacking when they have no hope to win, there are some who wander the borders of the kingdoms, waiting for an opportunity to strike. However, their instinctive hostility remains. Despite their apparent intelligence, older Grimm simply use it to become more effective in their singular purpose: killing.
It is commonly believed that Grimm do not require sustenance; when Grimm die, their corporeal form evaporates, leaving nothing behind and preventing detailed anatomical studies. Grimm usually die off if kept in captivity if they cannot kill their captors or escape; they cannot be kept alive by normal means. It is entirely possible that Grimm survive on negative emotions or the act of killing itself.