Hey all snakeonthebus here with another blog. Actually, now that I think about it I don't believe this is my first blog about our friend shrouded in mystery. Nevertheless, who better to write twice about than him. 
So, without further ado, let's delve into the subject of Herobrine. A name that has captured the attention of any and all Minecratians. Many, many skins, blogs, videos, etc. have been created surrounding the subject, but what do we really know of the infamous figurehead of chaos who clearly has a firm foothold in our sandbox.  
ORIGNS:
Herobrine first showed up on 4chan's /v/ board. It showed only one image of him and was not very popular. That is, until Copeland staged a scene on a livestream, photoshopping in Herobrine and pretending to be terrified of him.
THEORIES/CANONS:
There have been countless numbers of theories of how Herobrine came about. However, one portion that most everyone agrees upon is that he was created by none other than Notch. Which I suppose is a reasonable exploit. I imagine (and I am not the only person to suggest this part) Herobrine was supposed to be a sort of human AI. A prototype to the testificate, if you will. 
Now there is a major flaw in this theory. Namely, the Minecraft Wiki. Accroding to the website, there is nothing in the source code nor any code that would qualify an entity to possess properties of the fiend.
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Another popular, but ridiculous, theory is that Herobrine is Notch's brother who has died and was somehow placed in the game. The problem with this (among other things) is that Notch has no brother.
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There are some quite plausable theories in take.
One--the more likely--is that Herobrine is a virus, of some sort. He shows many characteristics of a virus. He has been known to change worlds, and even delete threads and send messages.
The second is that Herobrine is a figment of the players imagination. Personally, it seems unlikely, but I am here to tell you the facts and let you decide for yourself.
CHARACTERISTICS:
Like I mentioned before, Herobrine has characteristics of a virus. Here is a full list (of what I can gather) of what is said he can do:
-change worlds
-build temples
-create 2x2 tunnels
-shave off tree leaves
-post signs
-destroy blocks
-place blocks
-kill players
-teleportation/may be able to run 
-spawn dead mobs/control passive mobs
As for his physical appearance, there is little to note. Herobrine looks exactly like Steve, other than the suggestion that he has glowing, pure white eyes. He is also said to carry a pickaxe on occasion (another canon).
Herobrine is often considered in three different states.
Stalker: Herobrine follows the player and disappears if is spotted.
Violent: Herobrine attempts to trap player. He waits for players to explore the dungeon he created,  then either traps them inside or teleports behind them and kills them.
Griefer: Herobrine breaks players things and steals directly from their chests.
CONCLUSION:
Oh, there is also a fully registered player who goes by the name, Herobrine, so don't worry about him if you do happen to see the human, Herobrine. 

In conclusion, the only thing left is for you to decide for yourself. Does Herobrine exist?