Belter is a term used to refer to persons born in the Asteroid Belt or the Moons of the outer planets. In a pejorative meaning, they are also called Skinnies, because of their altered physiology due to growing up and living in a low gravity environment.
Background
They are an oppressed and mistrusted working class that provide many goods for Earth and Mars through the mining and other work facilities built on the asteroids. Although they are a weaker, less organized faction, they have started to resist the powers of Earth and Mars. The Outer Planets Alliance (OPA) was formed to bring together the Belt and to further their interests.
Physically distinctive traits
Humans born in the Belt are taller and thinner than those on Earth and Mars, standing 2-2.5 meters tall, and have brittle bones from growing up in low gravity. As a consequence, they cannot survive in Earth's gravity for very long unless they are in a specially designed water suspension tank. However, Belters recover faster than Earthers after periods of high-G. Belters also have elongated limbs and enlarged craniums. These physical differences along with cultural and language difference cause Belters to be dehumanized and regarded as second-class citizens by many Earth and Mars residents, since they superficially seem to be a deviation from the species norms of humans.
Despite this, it is still possible for Belters to mitigate the negative effects of low-g, particularly through exercise, steroids and bone-density enhancement therapy (a process that is known to be quite painful). Not all Belters are able to handle this adaptation; one known point of failure is during the improvements to the cardiovascular system (i.e. heart) as improved blood-flow is needed for surviving in full-g, as evidenced by several of the initial Belter colonists and Naomi Nagata during their first couple of days on Ilus IV. Belters that do hard physical labor, such as vacuum welders, tend to be stocky and raw-boned, while certain Belters are even described as muscular, such as Cyn (an OPA operative).
Language
Belters speak Belter Creole, a patóis of the languages spoken by the original settlers of the Belt. Depending on the social status of the speaker, this can be nearly impossible for non-Belters to understand. They also have a variety of physical gestures not shared by people from the inner planets, due to the need for non-verbal communication during spacewalks.
Gestation
Due to the effects on the human body in low gravity, Belters have great difficulty in completing normal, healthy pregnancies. Abdominal and uterine muscles atrophy from underuse, rendering labor a very lengthy, painful, and dangerous process. When possible, Belter women will relocate to Ceres station where the spin gravity allows a more predictable pregnancy and delivery. Ganymede station is frequently cited as a hospitable location for childbirth due to the high density of medical science professionals among the belt and outer planets.