Aleen is the homeworld of not one, but two sentient species - the garrulous and social Aleena and the mysterious, subterranean-dwelling Kindalo. The two species can co-exist due to what is effectively two separate biospheres - the forested surface world, and a second underground world, formed by the action of water, plate tectonics, and the deep burrowing roots of the arbozoic trees native to the world. The two sentient species are also kept apart due to their differing respiratory needs, each finding the atmosphere in the other's environment toxic to their biology. Very few have ever explored the Kindalo's dark world, and few if any records exist of it. Aleen's surface world, on the other hand, is a nature lover's paradise.

Over sixty percent of the planet is covered in forests, from cold forests of pines and other conifers near the poles to warm rainforests near the equator. Many species of herbivores and their predators roam these forests and the native Aleena evolved their renowned dexterity partly to avoid becoming a meal for these large carnivores. One peculiarity of the planet's terrain is that the land gently slopes from pole to equator, resulting in massive river systems and small oceans girding the planet. The plains of central Aleen are where the capital city, Reunion, is located, and the area of the planet most familiar to visitors.

Historically, Aleen was part of the Infinite Empire, the Aleena at the time being fierce and warlike servants of the Empire. The Aleena built towering monoliths to their masters and to the various religions they followed at the time. The remains of these monumental constructions still dot the surface of the planet today. In the past, Aleen was much more urbanized than it is today. However, following a long war between the two sentient species native to the planet that almost wiped out both species, the Aleena dismantled many of their cities and returned to a more pastoral lifestyle. Remnants of these cities can still be found, much decayed and overgrown with trees, home now to the local wildlife.