Heero is typically cold and stoic, fitting that of a battle-hardened war veteran more thn a fifteen year old boy. He frequently espouses the philosophy that being emotional on the battlefield will only result in heartbreak, so soldiers need to be able to fight without caring about their opponents. However, he believes that there's nothing wrong with showing emotions in normal situations.
Like the rest of the Gundam pilots, Heero was trained by one of the scientists that created the Gundams (in his case, Doctor J) to destroy the OZ foundation on Earth before they could stage their coup.
Season 2 Edit
During the battle, Heero and Starscream encountered Zechs Merquise in his Tallgeese, wanting to test it out a Gundam. In the middle of their duel though, Lady Une of OZ set a deadly trap in motion, threatening to use the Alliance's own missiles to destroy the Colonies unless he and the other pilots surrendered. Doctor J broke into the transmission and offered his own surrender, but not that of the Gundams. Following this implied order, Heero emerged from Wing's cockpit and activated its self-detonated device while standing on the hatch, destroying his Gundam and sending him hurling towards the ground. With the severe injuries he sustained and subsequent massive blood loss, he died almost instantly. His body was then recovered by Starscream, who returned him to base to be buried.