Aeryn Sun (played by Claudia Black) is a former Peacekeeper (a militaristic police force) pilot and Officer of Icarion Company, Pleisar Regiment. Although she appears to be human, she is in fact Sebacean - a species (practically) indistinguishable from humans. When John Crichton appeared through a wormhole, Officer Aeryn Sun was one of the pilots ordered to bring him in.
However, Aeryn's Prowler was caught up in the stream of Moya's starburst and was towed along when Moya first escaped from the Peacekeepers along with her crew. Aeryn was brought on board Moya as a prisoner, but because of her time spent with Crichton and the others, she was deemed "irreversibly contaminated" (a removal of name, rank, and status as a Peacekeeper) by Bialar Crais, and was ordered to be put to death.
She was in turn saved by the crew of Moya, and has since learned to think beyond the strict, militaristic confines of her Peacekeeper upbringing. She is very well skilled in hand-to-hand and weapons combat, and can more than hold her own in a fight. She has also become a valuable and important member of Moya's crew, and a companion and romantic interest to Crichton.
Aeryn was raised from birth to become a Peacekeeper, and she now has difficulty dealing with the fact she can never return to her previous life. Her experiences, however, have taught her that there is more to life than the emotionless, structured existence of the Peacekeepers.