Kynthia (a.k.a Kynthia O'Neill) was a native Argosian woman and the second wife of Colonel Jack O'Neill. Though she was programmed by the Goa'uld Pelops to live a life span of only 100 days by nanites in her system, thanks to SG-1's intervention in 1997, the nanites were shut down and she and the other Argosians were allowed to return to live at a normal rate.
Kynthia, like all other Argosians, had a lifespan of approximately 100 days. Because of the short lifespan, she (along with the other Argosians) blissfully celebrate everyday and be thankful to the "creator" (Pelops) for their one hundred days of living. However, their way of life was brought on because Pelops, a Goa'uld, had long ago set aside the people on Argos as "lab rats" (or "The Chosen" as the Argosians called it) to see how Humans would evolve by accelerating their growth via Nanites, thus shortening the Argosians' lifespans to approximately 100 days, or one 250th of the usual human lifespan.
Kynthia, like many Argosians, first encountered SG-1 of the Tau'ri when they were on mission to Argos in 1997, when she was 30 days old. After seeing Colonel Jack O'Neill, she decided to become his bride, so she presented him with a drugged cake, after which, while O'Neill was still under the influence, they consummated their marriage. The sexual contact made some of Kynthia's nanites transfer to O'Neill, which caused him to age faster than the Argosians. General George S. Hammond would then quarantine the planet as a result.
While Captain Samantha Carter, Doctor Daniel Jackson, and Teal'c of Chulak were at Stargate Command working on a solution, O'Neill remained on Argos with Kynthia. Kynthia attempted to comfort O'Neill, where she later learned that O'Neill was over forty years old, and became destressed at first at the prospect that humans can live for several decades, rather than 100 days; Kynthia has said that no one on Argos knew of any other kind of life and that they were supposed to celebrate each day. Later, O'Neill managed to convince Kynthia, along with the rest of the Argosians, that Pelops was not a god, but an alien who used them as test subjects and that he does not care for them. They succeed in removing the statue, breaking the technology which controlled their lives.
One night, O'Neill and Kynthia journey to the shore. He told her about his former wife and how it would take time for them to develop a loving relationship. Kynthia also blamed herself for taking his life from him and vowed to give some of it back. They spent the time he had left (it had been estimated that he only had about two weeks from the time that he was infected). O'Neill also taught Kynthia how to play Tic-Tac-Toe. They later realise long into the night that Kynthia was still awake (the Argosians would fall to sleep by sunset and wake up by sunrise because of the nanite programming). O'Neill deduced that there was a connection to their condition and proximity to Pelops' temple. He discovered the broken technology and it was determined that the device sent two commands via two separate frequencies; one to put them to sleep and the other to wake them up.
The rest of SG-1 returned to the planet and Carter broadcasted the frequency to wake everyone else up. From that time on, the nanites were disabled, allowing the Argosians to live at a normal rate, and O'Neill's aging would reverse within a few days. When saying goodbye, O'Neill promised that Stargate Command would come and visit and help them out. For his parting words, O'Neill said to Kynthia, "Sweet Kynthia, I've learned so much from you. I'll treasure every day of my life because of you."
A few months later, SG-2 had visited the Argosians again. Kynthia hoped that O'Neill could one day return. O'Neill also mentioned that, if faced with the possibility of retiring off-world, he would return to Argos. (SG1: "Politics").