According to wikipedia
GAT-X303 Aegis is a prototype transformable attack-use mobile weapon, it is featured in the anime series Mobile Suit Gundam SEED. The unit is piloted by Athrun Zala.
The Aegis is a commander-type mobile suit with very advanced sensors and heavy attack power. It can transform into a mobile armor configuration armed with a "Scylla" 580mm energy cannon (which is later mounted on the GAT-X131 Calamity Gundam). This cannon is powerful enough to destroy even a spaceship. Additionally, the suit also features a hand-held 60mm beam rifle. For close-range combat, Aegis is equipped with 4 beam sabers. As these weapons are mounted on the forearms and feet rather than hand-carried, all four can be wielded simultaneously, in either mobile suit or mobile armor configuration. Like most other mobile suits, the Aegis is equipped with two "Igelstellung" 75mm multi-barrel CIWS in its head. Lastly for defense the suit mounts a single shield on its left arm and is equipped with the Phase Shift armor system.
Designed as a command-type mobile suit by the Earth Alliance, the Aegis was one of the original five mobile suits created as part of the Atlantic Federation's G Project. The Aegis eventually was rolled out on January 20, CE 71, together with the other suits produced as part of the project. Stored in the colony Heliopolis, it became the target of ZAFT and, together with all but one of the prototype mobile suits, was stolen by them on January 25. Its pilot is Athrun Zala, who then used the suit for several months.
The Aegis is transported to Earth shortly after the Battle of Orbit on February 13 to pursue the Archangel. It is held at the Carpentaria Base in Australia until the Archangel passed through the Indian Ocean, where it was sent to rendezvous with the Vosgulov-class submarine Cousteau. The Aegis, GAT-X102 Duel, GAT-X103 Buster and GAT-X207 Blitz successfully intercepted the Archangel in midocean and forced the heavily damaged assault ship to lay over at Orb for repairs.
Finally, its destruction occured in late May while fighting Kira Yamato in the GAT-X105 Strike and Tolle Koenig in his FX-550 Skygrasper. Athrun self-destructed the Aegis while grappling the Strike, destroying both units. He ejected before self-destruction and was found injured on the shore by Cagalli Yula Athha. Later in that year, he returned to combat piloting the new, Aegis-derived ZGMF-X09A Justice. From June to the end of the war he piloted the Justice as part of the Clyne Faction, alongside Kira Yamato in the ZGMF-X10A Freedom.
The fragments of the destroyed mobile suit eventually found their final, peaceful use as a playground for young children, on the same island where Athrun and Kira fought. Its shield was later found and sold at the black market auction for junk where Lowe Guele picked up the TMF/TR-2 BuCUE Tactical Reconnaissance Type which head he used temporarily for his MBF-P02 Astray Red Frame. A variation on its transforming system is used by ZAFT, on a much larger scale, in the ZGMF-X11A Regenerate.
After the war, the Earth Alliance produced another Aegis which was then upgraded by Actaeon Industries into the GAT-X303AA Rosso Aegis.
Author p/s: In comparison with NG 1/100 Aegis Gundam, there were alot of huge differences in term of mobility of the modelling kits, the detailed of the armor, and the ability of transformation is another unique buying point for this gundam kit too, MUST BUY item!!