We are all psychopathic to an extent. Some are more than others.
Koji Kiriyama was a human male officer in the Stormtrooper Corps of the Galactic Empire, hailing from the planet of Coruscant. Growing up in Imperial City during the early reign of Emperor Palpatine, Koji was propagandized into enlisting in the Imperial Armed Forces. He took part in some of the Empire's early campaigns, later fighting in the Galactic Civil War. During the conflict, he witnessed Imperial atrocities and left the Stormtrooper Corps at the rank of major. After a period of remaining neutral, Koji joined the Alliance to Restore the Republic, around 2 ABY. Following the Battle of Endor, Koji served in the ground forces of the New Republic.
Koji was described to by slightly psychopathic, and had sadistic tendencies. He seemed to not care about the survival of his teammates. However, Koji nonetheless disliked what the Empire did, simply due to the large scale of it's atrocities. After serving in the Rebel Alliance up to Endor, he felt that the New Republic needed him, and continued serving as an army captain.
Koji was born in the Imperial City on Coruscant in 19 BBY, to Ishida and Haruka Kiriyama. His birth was just hours after the Declaration of the New Order and dissolving of the Galactic Republic. His father was the owner of Fregata, a rich corporation that specialized in the distribution of weapons and military hardware during war. His mother worked at the Internal Security Bureau, and it's successor, the Imperial Security Bureau. Living in a mansion, Koji was often ignored by his busy parents. He thus developed a very anti-social personality.
Despite this, he made on friend at his private school, a girl named Suzume Ujiyuki. Koji was a prodigy, and Suzume was the only one at the school who managed to keep up with him grade-wise. Koji was not particularly strong or fast, but exercised almost daily. Throughout his early life, Koji was exposed to Imperial propaganda, and attended at least one Empire Day Parade at the Imperial Palace. Suzume, who disliked the Empire, tried to get him to turn against the Imperials. However, Koji ended up enlisting in 2 BBY.