The great Kakashi ^^

Kakashi's background goes unexplored for the first part of the series, not being shown in any detail until Kakashi Gaiden, a six chapter series that divides the gap between Part I and II of the manga. During Kakashi's youth, his father, a ninja who was highly revered in Konohagakure, abandoned a mission crucial to the village in order to save the lives of his teammates.

 Disgraced by the villagers and those he had saved because of the repercussions the village suffered, Kakashi's father committed suicide. Wishing to avoid his father's dishonor, Kakashi adopted the philosophy that the success of a mission must always come first, becoming humorless and by-the-book in the process.

In later years Kakashi was assigned by his teacher, Minato Namikaze, to lead a mission that would turn the current war in Konoha's favor. When his teammate, Rin, was captured by enemy ninja, Kakashi elected to leave her behind and finish the mission. His other teammate, Obito Uchiha, rejected this alternative, stating that those who abandon their mission are trash but those who abandon their teammates are worse than trash and that Kakashi's father had done the right thing by saving his teammates.

Touched by Obito's words, Kakashi joined him in a rescue effort, though his left eye became damaged in the ensuing battle. After finding Rin and preparing to escape, an enemy-induced cave-in crushed Obito's right half. With his dying wish and the possibility of enemy reinforcements, Obito urged his teammates to leave and finish the mission.

 Before leaving, Obito had Rin implant his newly acquired Sharingan into Kakashi's damaged eye socket as a gift. His new eye in place, Kakashi fled with Rin as the cave collapsed, eventually completing their mission and forever mourning Obito's death.