Aasimars are descendants of humanoids and celestial beings, usually angels. Aasimar's come in many shapes and sizes, usually having the closer resemblance of their humanoid parent(s), with some twists that have been inherited from the angelic parent or ancestor. In some cases, the difference is so subtle, that most would not realise that they are dealing with aasimar at all.
Compared to tieflings, aasimars tend to be far more rare and hard to come by, due to their less obvious appearance and the nature of the angels that they descent from. Angels do not tend to bless humanoid lines with their blood or procreate with them to a similar degree to fiends, being far pickier about their chosen. This can majorly affect how aasimars are viewed, making them both revered and frightening, depending of whom you ask.
Curiously, aasimars don't tend to have such overtly distinct features as some of the other hybrid species, though differences between individual aasimar can be far more bizarre and striking. Some tells of the aasimar-blood can be metallic- or jewel-like colouring and reflection in eyes, skin or hair, rarely all at once, and sometimes in a very subtle manner.