How did Herobrine come about?
There is no way to pinpoint the exact moment the character appeared. The first reports from those who encountered the entity are very old and all claim that Herobrine appeared randomly, while the player was doing common actions, such as exploring and mining for items.

Once, a YouTuber called “Copeland” made a stream where the weird NPC made a special appearance.

Apart from these two sources (including 4chan's creepy-pasta) all information about its existence is very nebulous. And, let's face it, you can't 100% believe everything that is posted on 4chan.

Herobrine has a very similar appearance to Steve, another mob in the game, but with the difference of not having defined eyes – leaving just a haunting white look.

Minecraft developer Markus “Notch” Persson said that the bizarre character was never implemented and that he was removed in Beta 1.6.6, before the game's release. Yet people still encounter him from time to time.