Heero Yuy
The pilot of the Wing Gundam, the boy known as Heero is the most grim and serious of the Gundam pilots. His true name is unknown, but for this mission of revenge he has been code-named after the martyred leader of the space colonies. Raised by the scientist Doctor J, Heero has been trained since childhood to be a ruthless killing machine, equally proficient in mobile suit piloting, personal combat, demolitions, and computer hacking.
Though Heero's upbringing hasn't quite managed to squeeze all the compassion out of him, he is nonetheless callous towards his allies, and cold-blooded towards his enemies, and almost suicidally careless of his own life. His unexpected reluctance to kill Relena, after she discovers his identity, is the first step in Heero's gradual reclamation of his own humanity.
The erstwhile human killing machine has mellowed a tiny bit by the time Endless Waltz begins. Still, he keeps a close eye on current events, scanning the computer nets for signs of trouble and watching over his friend Relena from afar. When Relena falls into the hands of the Mariemaia Army, he immediately rushes to her aid.
Unlike some of his comrades, Heero was intentionally selected and trained for the role of a Gundam pilot. In an Endless Waltz flashback, we get a glimpse of how Heero was molded into the ruthless killer we saw at the start of the TV series. When a training mission went awry and resulted in innocent casualties, Heero's grief was seen as a sign of weakness. Dekim Barton, mastermind of Operation Meteor, ordered Doctor J to re-train the lad to eliminate these human failings.