-Who is he?
This mob is the famous Herobrine.
Most of the people think it is just a mod, and they are
right. This mob never was present in normal minecraft.
-How is the Herobrine mod?
This mod is only activated with a ritual, if you want to learn how to do it, just search in youtube.
-Does he apear in multiplayer?
Yes, but it isn't a CPU, is a player.
If you want to scare people out, open minecraft, the craked one, and wright "Herobrine" in the name:
Go to a server and start scaring !