From Ardrana

Just like tieflings, aasimar are plane-touched creatures that can’t quite be called Human. In their veins flows the blood of both humankind and one of the races of the Upper Planes -- the rilmani, the eladrins, or the guardinals.

Aasimar are beautiful creatures, with calm, serene features and an inner radiance that shines from their faces. They’ve got long manes of white-gold hair, and bright, piercing eyes that seem to look right through a basher. It’s easy to mistake an aasimar for a human of unnatural purity, a half-Elf, or even an agathinon. Aasimar tend to be noble, honest, and courageous cutters, but a body shouldn’t always assume an aasimar means him well; there are a few cross-traders and knights of the post among the aasimar, despite their noble birth.

Aasimar are scattered throughout the Outlands and Upper and Neutral Planes, but naturally avoid prolonged stays on any of the Lower Planes. (They’re too likely to be mistaken for an aasimon of some kind, and a tanar'ri or baatezu can’t stand the sight of an aasimon.) They usually dress to fit in with the population around them, so an aasimar living among the Elven folk of Arborea dresses like an Elf and assumes many of his hosts’ mannerisms. When an aasimar’s moved to great emotion, his heritage shines through his face like sunlight through clouds. There aren’t many evil bashers who can look an angry aasimar in the eye.

Note: Aasimar originally appeared in this form in the Planescape Monstrous Compendium Appendix II for the AD&D game from TSR. Their use here is for the purposes of providing context for the campaign only.