Well, first we should just clarify one thing ; he/it doesn't exist.
The legend starts when some random player was playing single player back in alpha. He noticed in the distance a player with the default skin barely in view, behind several layers of fog.
He chose to ignore it and continue with whatever he was doing.
A few hours passed and he saw it again, but this time he could make more out of it: default skin, but instead of having the regular eyes, the eyes were pure white.
This person decides to post it on the forums (minecraftforum.com). He wrote something like this:
"There is a player in the fog, I could have sworn I was playing single player, its starting to freak me out."
The post was removed from the forum rather quickly. So this person decides to post it again.
When he did, the post was removed quicker than the last. He posted yet again.
This time the post was removed almost immediately. But he got a message from the user Herobrine which read "Stop"
He went back to playing single player. He didn't see the mysterious figure.
Months later he noticed it again. But this time, it was destroying property. He wrote a reply to this "Herobrine" user which read "Stop why?"
He later got a reply which read "Ehhdjdjjjfbdvdh.... I am Not H's brother"
This person emails Markus saying "Do you have a brother?"
He later got a reply which read "Yes... but he is no longer with us..."
This player searched the skin of the user "Herobrine" which came up as the default skin, with pure white eyes.
Now, none of this actually happened, a group of teenaged boys came up with this story to scare people out of playing minecraft single player. This later led to mod creation and fan fiction.
