This is my theory on who the Purple guy is.
This theories hinge on the fact that Phone Guy never lies and that every little thing in the games is a piece of the lore
So, I believe there is going to be another game, and that will be the last one.
To explain why I believe this, I'm going to have to explain a lot of things. First of all, for me the timeline is going to be
FnaF4 > FnaF2 > FnaF1 > FnaF3
In FnaF2, the oldest on the timeline so far, Phone Guy tells us that there was a precedent location (Fazbear's Diner) in which something terrible happened that Fazbear Entertainment is trying to cover up.
We know from FnaF2 minigames that the secret is the 5 children that were murdered by Purple Guy. In Give Cake, we see the first homicide. Purple Guy killed a child outside the Diner during a birthday. This is how Puppet was born. In Save Them, we see Foxy witnessing the homicide the 5 children by Purple Guy (This explains why Foxy acts different from the other animatronics. While the others love children and all, Foxy is the only one who directly witnessed the murders). In Give Life, we see that Puppet put the bodies in the suits and as such haunted the animatronics.
The minigames are memories from the animatronics themselves. They have some degree of will and memory, love the children because of their programming and try to help them as much as they can. I believe this because the name of the minigames involves saving the children
The murders is the reason why the company spent some money on having facial recognition and a link to criminal databases. They don't want to repeat what happened at the diner.
The 5 souls that are inside Foxy, Freddy, Chica, Bonnie and Puppet are the souls from the children murdered at Fazbear's Diner. We know this because, in the minigames, the location is not one we've yet seen, and also because the Classic Animatronics are still walking around even though Phone Guy tells us that they tried to repair them but scrapped that idea because they were too ugly and smelly (caused by the children bodies inside of them)
So in FnaF2 we have two different sets of animatronics, the Toy set and the Classic set. They both have the facial recognition system installed, but while the Toy ones attack you from the get go, the Classic ones are not active until night 2.
on FnaF2's night 1, Purple Guy killed again. This made the children's souls active inside the Classic set and made them walk around even though they are so broken down. I believe this because on night 2 Phone Guy talks about something terrible that the locals are linking to the Pizzeria, to the point where it is closed down by the police on night 6.
I believe that the player in all FnaFs (except 3 and the minigames) is the same person and that person is Purple Guy. The Toy animatronics attacked the player immediately, but while active during another night guard's shift, never attacked him. They spooked him, but he didn't quit. Phone Guy takes the role of guard in FnaF, and also tells us so on night 6 in FnaF2. Why would he risk his life for minimum wage? he doesn't know what the animatronics are capable of.
Back to Purple Guy, he was convicted for the events at the diner, as per the newspaper clips that appear in FnaF1, but for some reason got out of prison because he turns up not that much later in FnaF2. This explains why the Toy animatronics are after him. He never meets any other employee, the owners are different (Phone Guy says on night 5 that they are trying to talk to the previous owner regarding the incident to track down someone, so it is different from the current owner). He knows his way around the animatronics. From the FnaF3 minigames we see him disassemble the animatronics pretty quickly. He has a spring crank and knows that Springtrap can be used as a suit. He knows about the layout of the restaurant. He even knows about the secret safe room where animatronics can't get to, which the public doesn't know about. This means that at least he was at the diner employee. Phone Guy also tells us on FnaF2 night 5 that the building is on lockdown because they are trying to avoid any past employees to ever coming back to the restaurant. He also (but this is a little shaky) tells us in night 1 that we "are golden" regarding the lights and that we should put on the Freddy head if we ever need it. This means that Jeremy is the only person that puts on a Freddy suit, other that Purple Guy.
So anyway back to FnaF2. At the end, Purple Guy is back on night 6 to the surprise of Phone Guy. He reveals that something horrible happened and that someone used a suit in the spare room to lure children inside and kill again. At the end of the phone call, he talks about a birthday in which Jeremy is going to be present as a security guard. I believe that in that occasion, Purple guy tried something but Foxy or Mangle (this would explain why the Toy set is discontinued but the Classic one are still around in FnaF1) sprung up and bit him, to defend other children. This is the infamous bite of '87.
In FnaF 1, only the classic set is around. Purple guy is back. He kills again on night 1, and the animatronics are ready this time. He defends himself until night 6, after which we see the events of the Good Ending. Purple Guy dissassembles the Animatronics, trying to destroy the children's soul. He instead frees them, and it is confronted by their ghosts in the last minigame. He knows he will be left alone if he enters one of the suits (because it worked in FnaF2), so he enters the Springtrap, and laughs because he believes he is untouchable. But the spring locks fail, he dies, he gets walled up by Fazbear entertainment and the children disappear. Also, on night 4 Phone Guy dies, so he can't be Purple Guy dies.
In FnaF3 SpringTrap's found and put in Fazbear's Fright. We play as a normal employee this time (we have no reason to believe otherwise) and the only animatronic is haunted by Purple Guy. He wants to kill everyone he can and this upsets the souls of the children. In the bad ending, we see that while the FnaF3 player escaped, the soul of the children are still inside the animatronics, but are in their own personal hell. They know that Purple Guy is still around and is capable of killing, but they lost all the ability to move around. There's only their heads, lying around or put up on walls, silent witnesses of a cycle they tried everything they had to stop. Look at their faces (particularly Chica) they are in distress while springtrap still lurks in the shadows
The nightmare ending shows us that Fazbear's Fright burned down, and the good ending (obtainable only after the nightmare ending) shows us that the souls are at peace, and that Purple Guy is gone.
This is the end of FnaF, but I believe Scott is screwing with us in the same way he did in FnaF2. This is the last game in the timeline, so what happens in the Good Ending is truly the last thing that happens to these characters, so it is The End, as Scott puts eloquently over the animatronics' heads
but there are too many unanswered questions. What are Purple Bonnie and Purple Freddie? what's going on in FnaF2 with Ballon Boy? Why does the music keep Puppet at bay?
I believe there's still a location we have not visited, and the answers are there. I have other things to say about the games, but I left them out because the post is long enough already
TL;DR IT'S US, WE CAME BACK, WE ALWAYS DO
I also borrow heavily from two theories I saw on Youtube that I'll link in a moment