Grimer is a slimy, amorphous Pokémon, born from sludge in dirty streams or seabeds that are exposed to lunar radiation. It has two large eyes with beady pupils and a gaping mouth with a gray tongue. While it lacks any visible legs, it does have two arms with three digits on each hand. Its purple sludge body contains a variety of pungent bacteria, which are so potent that it renders soil barren and incapable of supporting plant life. It is constantly oozing a bacteria-rich fluid from all parts of its body. Should it run out of internal germs, Grimer will die. Pieces of it often break off during travel and go on to become new Grimer. When combined with another Grimer, it produces new poisonous compounds.
Due to its lack of a solid form, Grimer is capable of squeezing into any space or crevices with relative ease. It uses this to penetrate sewage pipes and feed off of the filthy wastewater inside. Its population fluctuates based on the availability of waste, which is Grimer's main diet. It can be found in polluted lakes and streams, as well as within cities and factories where trash and industrial waste can be found. In recent years, thanks to efforts in environmental improvements and a decrease in their food source of waste, Grimer is now facing extinction.
Grimer was brought to Alola to deal with the excess waste generated by the region's increased population. Alolan Grimer eats solid garbage instead of sludge; this new diet has caused its body composition and form to change. Alolan Grimer's body is green with a line of yellow sludge along its lower lip that extends around behind its head, a bright blue tongue, and small black crescent-shaped spaces behind its eyes. It also has two blunt "teeth" in its upper jaw, which are actually crystallized toxins formed by its diet. It is not known how to break down these toxic crystals, but direct contact with them is known to be dangerous. Lethal poisons leak out of Alolan Grimer's body when these crystals fall out. Because it produces and stores toxins within its body, Alolan Grimer does not produce a foul stench like Kantonian Grimer.
There are usually about one hundred Alolan Grimer found living near waste-disposal sites in Alola, assisting people in dealing with garbage. Alolan Grimer is perpetually hungry and must constantly eat trash. If no trash is readily available, it will eat other manufactured products instead.
In addition to garbage, Alolan Grimer preys on Trubbish and Garbodor; its introduction to Alola caused their populations to decrease in the region.
In Go for the Golbat, Koga wore a Grimer on his shoulder in the form of a ninja outfit. He used it extensively to smother Blue, preventing him from breathing and restrict his movement so that he would be unable to call on his Pokémon. He used it again during the battle against Agatha, but to little effect.
In Sparks Fly for Magnemite, Grimer's anime debut, Grimer had a very strong odor that made most people around the Grimer cover their noses from the smell. However, in later episodes, Grimer's smell has seemingly ceased to exist. This is most obvious in Camping It Up!, where Dawn is given a Grimer to train, and it immediately hugs her, but no one complains about any smell.
Grimer shares similar traits to Koffing and Trubbish. All three Pokémon are pure Poison-type Pokémon with a single evolved form, roughly the same base stat totals and evolution levels, and appear to be based on a different kind of pollution; Grimer being water pollution.
Grimer appears to be a pile of living sludge, and may be based upon the general concept of living slime in popular culture. It may also be based on the imagined fear that heavy pollution, caused by the leakage and/or improper disposal of toxic/radioactive waste, may result in the creation of new and undesirable life. It may also be based on Dorotabō, a one-eyed, three-fingered yōkai that rises from the mud of neglected, overgrown rice fields.
Alolan Grimer may be based on an oil slick, particularly the rainbow colors that are produced when a thin layer of oil floats on top of water; this is due to a phenomenon called thin-film interference. In the real world, oil spills are a major threat to ecosystems in the world's oceans and particularly in Hawaii, which Alola is based on. Its reduced size may reference insular dwarfism or, since its Alolan Form was brought about by human influence, dwarfing.
Grimer may be derived from grime (dirt or filth).
Betbeter may be derived from べとべと betobeto and べたべた betabeta, both meaning sticky.
↑Samson Oak: "Did you know that Alolan Grimer are in fact not native to the region at all, but instead are what we would call naturalized Pokémon? There is not so much sludge and pollution for Grimer to feed on here in Alola, though, so they have grown to feed on garbage instead. They do not give off a foul stench, thanks to the way they contain any poisons within their bodies, but their crystals are terribly poisonous!"