The Xolpe are a species described in Matter living on the planet Bulthmaas. They have not yet attained Involved status and appear to have a technological level similar to contemporary Earth, but are under the 'mentorship' of the Nariscene.
The Xolpe are a humanoid race described as very short, squat and pale, possibly due to the high level of gravity on Bulthmaas (although both the Xolpe and their environment are only described from the perspective of a Sarl character, which may not itself be quite the 'average' humanoid norm). As Sarl visitors are still able to wear uniforms designed for Xolpe, albeit looking ridiculous due to the poor fit, it could be assumed that they don't deviate enormously in stature or shape from other human species.
The land masses of Bulthmaas have been left largely uninhabitable by a wide scale nuclear and biological conflict, leaving the surviving military powers to continue the war from large bunker complexes connected by underground train tunnels deep beneath the scorched surface.
The Nariscene claim to be helping, though it is later implied that they arranged the war on purpose so that - having long ago abolished conflict on their own worlds - they could experience warfare vicariously through another species using nano-scale surveillance devices. Their society is shown to be militaristic and authoritarian, though it is not stated whether this has always been the case or a reaction to the current state of war. Visitors to the planet are required to don uniforms at all times and carry identification papers and access to the surface is very limited, although dirigible hospital ships floating above the war zone exist to tend to casualties.
In the novel only one Xolpe nation (Prille) is described and their enemy is not named, though it would appear that much (or all) of the rest of the planet exists in a similar state of devastation.