Callisto is the fourth moon of Jupiter, about the size of Mercury but only a third as dense. Due to its relatively far distance from Jupiter and the presence of both deep-liquid and surface-solid water, it was chosen as an ideal colony during the early days of humanity's exploration. While never terraformed due to various reasons, it boasts many large colonies sheltered in its craters and a formidable orbital defense shield to protect it from meteors or unwary bullets.
The surface is very similar to Earth's moon, Luna, in some ways - a barren wasteland of white. However, Callisto's surface is composed just as much of ice as it is of rock, with large glaciers of dry ice and other more unusual things. It is unnecessary to say that this is highly hazardous and nearly impossible to survive unprotected. The surface is also heavily cratered- So much so, in fact, that there is virtually no where on the surface that is not a crater, save for inside of the colonies (on good days). Most of the cities make use of the deeper craters to shelter their populations from the harsh radiation emitted from Jupiter, thus eliminating the need for lead-lined clothing and replacement reproductive organs that form such a booming business elsewhere.
Callisto has a notable rivalry with Europa, due to the fact that both continue to claim (despite no evidence having been recovered) to have the only non-Earth life in the Sol System, among other factors.
Callisto has a long history of living up to its mythological namesake. Its training grounds are well-known for both their harshness and their quality of soldier, lending those trained their the nickname of 'Bears'.