Aasimar are a race driven by the memory of what they once were. In the time before time they were angels, immortal servants of the gods of good who chose to be bound in flesh to wage an endless war against darkness. They also live in fear of what they may become. An aasimar who gives in to evil transforms into a fiendish rakshasa.
The aasimar do not reproduce like other races. Instead when one dies she is reborn as a full grown adult someplace else in the world. A newly "born" aasimar has no memory of her previous life other than a few hazy glimpses of what she is and what her mission is. Other than that, she only retains enough memory to speak, care for herself, and give praises to the gods.