Ok, so Herobrine is def NOT real, but there are other Herobrines that aren't actually Herobrine. There's also Entity 303, Norm, etc.. Most are hacks from fired Mojang programmers. They have appeared in a number of gameplays, live recordings, or even just innocent gaming. These mostly happen at 3:00 AM. We don't know how to destroy these hacks, so just play Minecraft at 3:00 AM unless you want to have your computer/console/phone crashed or possibly destroyed.
*Oh, and P.S., if you find a cross sign, turn off your phone/computer/console.