In Minecraft does herobrine exist in the game?

Yes, I've seen him before.

No he doesn't. The game has been decoded numerous times by coding experts and there's not 1 single code that links to an existence of such character. He can only exist through plugins and mods, if you play online then you may see Herobrine game modes that you can play.

Yes he does actually. If you check the new update on minecraft, it's says: GOT RID OF HEROBRINE. Well that's not true. Actually, I've kind of in countered him a couple of times and it can be scary. How to know if HEROBRINE is in your world:
-Random Structures
-You get splashed by potions randomly
-You keep taking damage for no apparent reason
-Your house is destroyed

If you see anything like this, delete your world. It's the only thing you can do. Don't hit him or destroy his structures or it's not gonna go well for you.