Sai Argyle

History
At the beginning of Gundam SEED, Sai lives on the Orb space colony(which was neutral in this series) of Heliopolis with his family; friends Kira Yamato, Tolle Koenig, Miriallia Haw, and Kuzzey Buskirk, with whom he is enrolled on a robotics engineering course at the local college; and fiancee Flay Allster. His peaceful life ends when ZAFT raids the colony in search of prototype EA mobile suits which have been secretly produced there. As the fighting ultimately destroys the colony, Sai and his friends end up as refugees on the EA battleship Archangel. When Kira is unofficially drafted to pilot the remaining GAT-X105 Strike, Sai and the others (except for Flay) decide to help out. They are assigned to various bridge positions; Sai is in charge of enemy detection and electronic warfare identification on the CIC of the Archangel.

When a twist of fate lands PLANT pop idol Lacus Clyne on board the Archangel, everybody (except for Flay) is mesmerized by her kindness and beauty. Although Sai is respectful of Lacus, he does question whether the reason that she has such a lovely singing voice is due to her genetic alterations. He and Miriallia help Kira when he elects to return Lacus to ZAFT and along with Tolle are understanding upon learning ZAFT's Aegis pilot Athrun Zala is a childhood friend of his.

When the Archangel finally meets Admiral Lewis Halberton and his Eighth Fleet, Sai and his friends are given discharge papers. However, Flay finally volunteers to enlist, saying her father's death has made her realize what is going on and that she feels she must do something about it. Sai tears up his papers and says he feels the same way, and the others follow suit.

End of relationship with Flay
Flay's true motive in enlisting in the military, however, is to manipulate Kira into fighting the Coordinators on her behalf until he died in battle. She blames her father's death on him thinking {correctly} that he deliberately held back while fighting the ZAFT forces that ultimately killed her father. In North Africa, Flay breaks off her relationship with Sai in favour of Kira. Enraged, Sai lunges at them, but Kira easily overpowers him. This prompts Sai to try moving the Strike Gundam, but he fails. As punishment for doing this without permission, Sai is put in solitary confinement for a week. Despite his humiliation, Sai doesn't try to win Flay back and tells Kira that he (Sai) is counting on him. In the ensuing skirmishes, ZAFT pilot Nicol Amalfi is killed by Kira, Tolle is killed by Athrun, Kira is reported MIA, and ZAFT pilot Dearka Elsman is taken prisoner and his GAT-X103 Buster recaptured.

With Kira gone, Flay attempts to restart her relationship with Sai, denying she ever felt any emotions for Kira. The arguing former couple encounter an enraged Miriallia trying to avenge Tolle's death by killing Dearka Elsman, who was being held prisoner on board the Archangel. Sai stops Miriallia and calms her down, then Flay states her belief that Coordinators must be eradicated. She attempts to shoot Dearka, but is knocked down by the recomposed Miriallia, Flay demands an explanation, stating that she and Miriallia are now the same, an opinion Miriallia vehemently denies.

Later
Later, Flay is transferred from the Archangel, only to be captured by ZAFT Commander Rau Le Creuset during Operation Spit Break. Meanwhile, Sai and the rest of the crew defect to Orb after they are saved by Kira in the stolen ZGMF-X10A Freedom when the Alliance leaders attempt to sacrifice their less desirable forces, including the Archangel and its crew to wipe out a majority of ZAFT's forces. Sai makes peace with Kira, and the two of them remain on good terms for the remainder of their association.

When Kuzzey Buskirk complains that people will think he is a coward for leaving the Archangel, Sai tells him not to worry about what other people are doing and just stick by his decision.

Sai continues to serve at his post on the Archangel throughout the rest of the war. During that time, he realizes Flay still cares for him in some respects when she calls his name specifically while trying to communicate with the Archangel, and he is deeply saddened when Flay dies at the hands of Rau Le Creuset.

A short OVA set after the series shows him going back to a civilian life, one that apparently involves computers. He still keeps pictures of himself with Flay, despite their tumultuous history and seems content with this new life. He does not appear in Gundam SEED Destiny nor is he mentioned.