Theory: Mike is the Bite Victim v3.0.

Welp, third time's the charm.

Before I start this, I'd like to give a quick message to all of the people who read the previous 2 versions. Thank you for all the support you gave me. I never thought a theory like this would get so much acclaim.. on DA at least. Outside of here, people have been much less accepting of this theory, and I can see why. Admittedly, the previous 2 versions weren't very good. I'm still proud of the second version, but I feel like I should've gone more into detail, left out the stuff about the dream theory and thought up a better explanation for why the bite victim didn't die. However, thanks to people like you, I've made the theory much more believable and likely. I'd prefer everyone go into this with an open mind though, since a lot of the new proof might contradict minor parts of the previous theory, like when the nights of 4 took place.

I'd also like to apologize to certain people who I tried pitching the theory to before. I was a lot more salty, close minded and arrogant back when this was a new theory, and I feel deeply ashamed of how I acted before. Hopefully this helps redeems myself in your eyes.

Part 1: Connections between 1, 3 and 4

I'm starting with this just so people can understand why I'm making such a connection in the first place. I'll get to the actual stuff regarding Mike in a sec.

The first connection between these three is the gameplay. Both 1 and 4 involved looking into doors and shutting them to prevent intruders, while also keeping an eye on one specific animatronic and making sure he doesn't escape, the animatronic in question being Foxy. I know this might not seem like much, but it becomes slightly more clear when you look at the rooms themselves. In both the first and fourth games, the hall to the left's most notable features are pictures. In 1, it's drawings (supposedly) made by children, while in 4 it's family portraits. While this might be a stretch, in 1 the east hall also had 2 large newspapers, while in 4 there are 2 white windows in the same spot.

Next, as pointed out by several people on Freddit, in the third game there were 4 animatronic "skulls" lying around the building. Bonnie, Chica and Foxy had their eyes lit, while the Freddy one wasn't. In the fourth game, the Bonnie, Chica and Foxy bullies had visible white eyes.. while the Freddy bully, for seemingly no apparent reason, lacks them. Coincidence? ....probably, but it's still worth point out. Also, as pointed out by my good friend , the minigames in 3 had a lot of parallels to 4. In BB's Air Adventure, you can find a random tree at one point, possibly the one from Fun with Plushtrap. There are also numerous windows everywhere, again like the one in FWP, as well as this.

Part 2: Reasons the bite victim lived

This is the part absolutely nobody liked. Everyone kept on insisting he died because of the flat-line, acting as if it's fact he died.. when it's not. Flatlines don't actually mean death, it means they can't detect a patient's heart. If the patient is taken off the device and they kept it running, it would flatline. That's not all though. As many know, one of the few Easter Eggs in 4 were pills on the bite victim's night stand. IRL, sleeping pills have been used to wake people from comas. Also, though a stretch, it's worth noting that there's still gameplay after the flatline. I know what you're saying, "Nightmare represents the child's death", but think about this. If one of the nightmares catches you, the game ends. What's the difference between dying at Night 2 and dying at the last cutscene? And how can you die when Nightmare is around? If he represents the player's death, how come you can "beat" him?

Before you ask, no, I'm not disregarding the flatline. I'm not gonna say it equals him simply leaving, there are much better ways of showing that. In fact, I DO believe it has a point. The bite victim DID die.... his old self, that is. More on this later.

Part 3: Mike 

Think about everything we know about Mike. He's male, has blue eyes, possibly of German descent, spent Five Nights at Freddy's, had severe hallucinations and was 100% mute. Firstly, think about the hallucinations. Note how the second game technically has none. All of them can be explained as the ghosts trying to show they're ghosts by moving stuff around, or that the hallucinations are lore hints and not actual hallucinations. They never actually "create" anything in 2, they just use what already exists. In 1, we see posters change and made into 2 things: signs saying "IT'S ME", and posters of a crying child. Take note that "IT'S ME" only shows up in the first game. Literally no signs of it in any of the other games.. save for the fourth. Note how there's an it's me sign in pirate's cove... PIRATE'S cove, and the crying children posters are only the head of a child, on a black background and.. well.... crying. From now on I will be known as captain obvious.

Now let's talk about Golden Freddy (or at least Classic GF, not W. Goldie). His only appearance on any of the cameras is on a poster, before he immediately comes into the office and crashes the game.. but not before "IT'S ME" and pictures of Freddy and Bonnie flash onto the screen. Think about it for a minute, the first game has a lot of bias for Freddy and Bonnie. As noted here, they're the only ones in any of the drawings, and the only ones in the first game to have eyeless screens. But why specifically them?... could it be because of these two?

The point I'm trying to make is that Yellowbear represents Mike slowly regaining his memories. The game crashes because he blacks out of fear and shock. "But wait, why would he come back to the place? He should know it's dangerous regardless of his memory or not" I have an answer to that too. As TvTropes pointed out, the song Les Toreadors is used, implying Mike is a "Nightmare Fetishist". Now, what's one thing people without frontal lobes can't feel?

Alternatively, as my friend theorized, it could be a family business. Phone Guy is Mike's father, and once he died Mike was given the job. Lacking any fear would certainly help.

Part 4: More about the bite victim

Nightmare is a literal mirror of Yellowbear. Instead of yellow body with black clothes, he has a black body with yellow clothes. Instead of standing still, he's all over the place. Instead of representing a memory gain, he represents memories being lost. That's why he comes right after the flatline. The flatline represented his old self dying... and so does the chest. "Somethings are best forgotten" is referring to the bite, his brother's bullying and the events he saw, the latter of which also being how it's "the pieces put together". "For now" is referring to Yellowbear and something else I'll get to in a minute. "I will put you back together" is the plush saying he won't have to live a horrible life anymore. He'll forget all the trauma he had and his brother is presumably nicer (if "we're still your friends" is anything to go off of).

Part 5: The Third Game

This part will probably be controversial for one main reason: it relies on the bite victim being friends with the missing children and Puppet haunting Plushbear theories. If you don't believe in either of those, don't quit just yet and listen to what I have to say.

When Fazbear's Fright was opened, the Puppet had sent visions to Mike, asking him for help, since he previously befriended him and attempted to help him back in 198X (the year doesn't matter in this theory), and he was also friends with the other 5 kids. What are these visions? Simple, the cutscenes from 2. This is why Goldie and the Puppet are in there. Not much to really say, but I'll show some proof of this later.

FNAF 3 has the most hallucinations out of any FNAF game. By now, he's obviously remembered his past, but due to being older he's more mature and accepting of it. The Spring Bonnie posters are his memories of the missing children incident. They disappear when Springtrap himself is in the camera because, well, Springtrap is the killer himself. The cupcakes are him remembering his last shift at Freddy's, or possibly his birthdays.

Part 6: The Final Proof

• "4 Games, 1 Story"
  
• The Paper Pals. Take note that the human one isn't called BB or child, but rather "buddy". He's also the only plate that moves in 2, the only game (hypothetically) without Mike. In 3, the only game without a proper Freddy or Bonnie, guess which plates appear instead?
• Take note that, while Fritz and Jeremy express some emotions, Mike expresses none and the 3 guard's only noises are him trying to breath.
• The phantoms could each represent something about the bite victim. Phantom Chica is Tiny Toy Chica, Phantom Foxy is the brother, Phantom Mangle the broken Mangle/decaying Foxy toy (that ImSmasher thinks is somehow the real Mangle), Phantom BB either the kid on the street or N. BB (or both), Phantom Puppet could be real (thanks to the cutscenes) and Phantom Freddy is Fredbear (hence why he's Golden Freddy)
• Speaking of the phantoms, some of them match up with things from the cutscenes. Namely, both Puppets stare and follow you, and the first frame of P. Chica's jumpscare is in the cutscenes.
• The eyeless screens from 2 could possibly help support this theory. Withered Freddy represents Fredbear as in 1, while Foxy is the brother and Toy Bonnie is the "Toy Spring Bonnie" head from 4. Why that? It means that's the suit the killer used.
• As suggested to me by /u/Doowopasaurus, the minigames from 3 could also potentially represent him regaining his memories, and by extension, freeing his friends.
  
• In the files of the second game, there exists a file of a skull....... labeled as "Mike_Skull".

And with that, my theory is now done. Thank you everyone for all the support, it really means a lot to me. If you don't agree with the theory, that's fine. Just remember to be considerate, respectful and give constructive criticism of the theory.