Kira is the main character in the first series. We meet him on Heliopolis in Phase-01, where we learn that he is just a normal kid going to school like everyone else. The story begins as Heliopolis is raided and attacked by ZAFT, causing Kira to reunite with his childhood best friend Athrun Zala. He ends up on the OMNI battleship Archangel with a few of his classmates, and becomes the pilot of the Strike Gundam—the only one not stolen by ZAFT—out of necessity.
Kira is the reluctant hero of Gundam SEED. He's a little clumsy and unsure of himself, but in the end he triumphs over nearly everything he encounters; without him, there would simply be no show. In the beginning he is firm in his decision not to become part of the war, but he soon realizes without him his friends are in danger. Mwu tells him that only the two of them can protect the Archangel, and that he should do what he has the power to do - pilot the Strike Gundam.
It would take entirely too much time to chronicle all that happens to him in the series, since he is the main character. But the core of it all is, Kira is just a normal kid who wants to live a normal life, but for reasons beyond his control he is thrown into a war. Throughout the course of the series he goes a lot of trauma, and it shows even in the post-series five-minute OVA, "Hoshi no Hazama de" (Between the Stars)—he's very quiet, and he wonders out-loud to Athrun why he's still here, eating and sleeping, still alive after everything that's happened and everyone who's died. The scene shows that it could take a while for him to sort out everything that happened and learn to forgive himself; but as Lacus told him earlier in the series, there may have been many people he's killed, but surely there are just as many if not more people that he has saved.