Okay, I've put much research into this.
Disc 11 Edit
Created by C418 disc 11 depicts a backstory of Herobrine, as told by the developers, so this probably is not helpful to who he actually is. It starts with heavy breathing and sounds of a cave, next we hear a (most likely) flint and steel ignite, and a few coughs. We then hear what sounds like a book open and the (most likely) flint and steel light again, then we hear a strange noise and walking followed by running, and the sound of dirt being placed. We also hear what is most likely wolves' howl, and then the disc ends.
Herobrine's Fans' Thoughts Edit
This is Herobrine's fans' thoughts/theories on Herobrine. Herobrine Is Notch's Dead Brother: Many people thought Notch's brother was Herobrine, so they reached out to him on Twitter, Notch responded by saying: "I did." which lead many people to believing he was Notch's dead brother. Later, Notch denies this and says that he was only joking around because of Herobrine's popularity, many still believe he has a dead brother, many believe it's just a joke. Herobrine has red eyes: Fans of Herobrine believe that Herobrine has red, bloody, eyes, Herobrine with red eyes was first shown in many Youtube Videos online, some fans believe this is true, some don't. Herobrine isn't evil: Some people believe that Herobrine is not evil, I believe that someone at Mojang (Probably Notch) said that Herobrine exists, but he's not out to hurt you, which could just be a way to get people to stop being afraid of him since telling people he does not exist does not work. Many Youtubers also use the "Herobrine isn't evil" theme in their Youtube videos, further spreading this theory. Many people believe it, and some don't. Herobrine is removed: Herobrine is said to be removed from Minecraft because Mojang put in their update changelog "Removed Herobrine". Only a few people believe this, most people believe he is still out there. Herobrine is in the nether: Many Herobrine believers believe Herobrine is in the nether, and by visiting their, and searching for him, they can find him somewhere. Many people believe this, but a few people don't. Herobrine places signs, redstone torches, netherrack, glowstone, take all the leaves of trees, etc.: A lot of people who believe in Herobrine believe that if Herobrine is in your world, he will remove all leaves from trees, place signs saying a message, and place redstone torches, netherrack, or glowstone, and many people will not believe Herobrine is in your world if these things are not in it, some people believe this, some do not. Herobrine can be summoned by alters or commands: Many people believe Herobrine can be summoned by commands (Such as /summon Herobrine) or alters (Like a gold alter with redstone torches and fire). However, they believe the only way to do this is to have the right circumstances, such as rain, or lightning. Many people believe this, may don't. (Feel free to add to this, it's very unfinished, remember not to be bias about it.)