Does Herobrine exist?

Notch on Twitter is pretty clear on the topic:

I have no plans of adding herobrine.

_Notch

Short Answer: No.

Long Answer: NO.

And he "removed" him in the 1.6.6 update.

No. While modding I must have seen every source file since Alpha 1.0_17 a couple of times, and there's simply nothing there. Unless he's an emergent feature of the deepest bowels of the rendering code.