Aeryn Sun is a former Peacekeeper pilot and officer. Although she appears to be human, she is in fact Sebacean, a species indistinguishable from humans in external appearance. At the time John Crichton appears through a wormhole in the beginning of the series, Officer Aeryn Sun is in her Prowler battling to retake the Leviathan Moya, which has been seized by escaping prisoners Ka D'Argo, Rygel, and Pa'u Zotoh Zhaan.
However, Aeryn's prowler is caught up in the stream of Moya's starburst and is towed along with the escaping Leviathan. Aeryn is brought on board Moya as a prisoner, but when she and Crichton encounter her commanding officer Captain Crais, she defends John from Crais' allegation that John deliberately attacked and killed Crais' brother, saying he isn't brave enough or smart enough to have done so. In return, Crais declares her to be "irreversibly contaminated" from spending too much time with an unknown life form, which means a death sentence. John Crichton and D'Argo soon escape the Peacekeepers again and, having nowhere else to go, Aeryn reluctantly becomes a fugitive alongside them.
Aboard Moya, she learns to think beyond the strict, militaristic confines of her Peacekeeper upbringing. Born in service aboard a Command Carrier, it is the only life she has known, and thus is very well skilled in hand-to-hand and weapons combat. She also becomes a valuable and important member of Moya's crew, and a companion and romantic interest to Crichton. It is only after leaving the Peacekeepers that she can begin to find out about her parentage. She has discovered that her mother, Xhalax Sun, was a Peacekeeper pilot who—against Peacekeeper regulations—fell in love with an older officer, Talyn Lyczac. They deliberately had Aeryn, a child born of love, and Xhalax sneaks into Aeryn's dormitory one night to tell her so, which is strictly against regulations. To redeem her crime, Xhalax was forced by the Peacekeepers to choose between the two, and she executed Talyn so that Aeryn could live.
Aeryn evolves from a cold, detached soldier into a valuable friend and crewmate. Her relationship with John Crichton also evolves, and Aeryn ultimately becomes a compassionate wife and loving mother. They name their son D'Argo Sun-Crichton, after their dear friend who seemingly lost his life in the Peacekeeper Wars.