I know this will not help you at all but this is the facts:
Herobrine is Notch's "brother" that died
Notch did not put in any coding of herobrine
There are still sightings of him when using mods.
I do not believe in him, because I've never seen him, no one I know has ever seen him, and Notch did not put in any coding of him.
Notch is furiously entering the text "*REMOVED HEROBRINE*" to stop people randomly spamming posts/videos about "sightings"
A lot of people will believe in him, but that's fine with me. They can believe what they want, I'll believe in what I want.