[Sazelyt]

Originally Posted by NoSanninWa

Spoiler for manga:

He stole someone's eye.

Sorry, I need to correct that statement.

Spoiler for manga:

He was willingfully given that eye by the original owner as a present, he didn't steal it. 4 posts above summarizes the events taking place well.

Originally Posted by Kekkai Genkai

2. What are "Gaiden's"?

Spoiler for manga:

Kakashi's Gaiden is about Kakashi's past, and it tells us the events taking place when he was promoted to Jounin, when he was still a member of Yondaime's team. That events include a mission that results in the death of his Uchiha friend (Obito), although Kakashi doesn't really consider him as a friend until the mission becomes saving another friend. It also tells us about how Kakashi obtained the sharingan eye. Another surprise of the Gaiden is Yondaime. And, the Gaiden gives a brief intro for Yondaime before he became a Hokage.

In short, Kakashi's Gaiden explains us the events that have caused significant changes in Kakashi, both physically and psychologically.