Okay so... The solo character thing has been an incredibly difficult topic to solve since it was brought to us, even if it isn't supposed to.
We've had a lot of hints about who it is throughout these months in The Official JOLLY Server, we have so many hints it's already difficult to keep track of every single one of them to solve this puzzle.

But today, I present to you the theory I think will put the pieces together in the best way possible, and with some more luck, finally solve the Solo Character mystery.

Jack Carson is JOLLY 4's Solo Character

First of all, I want to thank @Spacejunx for making me see all this Solo Thing from a different perspective sooner than i would've done on my own.

Second, I want you all to know the hints about the solo character that I used to build up this theory, these hints were provided directly from Weshek in The Official JOLLY Server.

• The Solo Character has been seen previously in JOLLY 3 Chapters 1 and 2 (More noticeable in Chapter 2).

• The Solo Character is a domesticated animal.

• The Solo Character is an animal which is impossible not to see when you go out.

• The Solo Character's color was originally almond white, but at the time JOLLY 4 happens, its colors are almond white and red.

• The Solo Character is a very smart being.

• The Solo Character was created in May 22, 1996.

• The Solo Character is an antagonist, but it's just a threat, it doesn't hurt you.

• The Solo Character wasn't an antagonist when it was first seen in the previous games.

• The Solo Character does NOT have an endoskeleton.

• The Solo Character's eyes are blue.

• The Solo Character has some kind of moustache.

• The Solo Character has clothes.

• The Solo Character can swim.

• The Solo Character would be destroyed by the Goodtime animatronics if they were on hostile mode.

• The Solo Character was once seen on TV, the people who saw it were either sad or didn't care.

• The Solo Character is the reason for all the events of JOLLY 4.

• The Solo Character was created by 2 people called "Bill" and "Ellis".

Of course, these are not nearly all the hints that have been given, but they are the ones I considered while making this theory.

Now, let's get to the theory and proof themselves.

We could start by confirming that us, people, are animals, Homo Sapiens Sapiens to be more exact.

So a human character could fit The Solo Character profile.

Now, we know The Solo Character was created in 1996, so, at the time JOLLY 3 happens, it would be around 21 years old. Just like Jack Carson at that time (Confirmed in The Official JOLLY Server).

We also know that Jack Carson is a white man with blue eyes (The first thing is evidentiable in the game, the second thing was confirmed in The Official JOLLY Server).

So, just as mentioned in the hints, Jack and The Solo Character share colors.

They both also wear clothes, and they both are smart, how do I know Jack is smart? well, he survived 2 weeks inside an abandoned factory with deadly robots by himself.

Both The Solo Character and a human can swim, and, they both also are a target for a Goodtime animatronic in its hostile mode because an ordinary unidentified human is something a Goodtime animatronic could kill.

Bill and Ellis could be Jack's parents because of the "creation" thing, which could easily be intercourse with a different name.

Now we get to my very own assumption about The Solo Character.

The Solo Character is some kind of "Jack Carson's reanimated corpse". Yes, a corpse [ this theory assumes that Jack Carson died in the AJMF (JOLLY 3 factory) collapse], why?
Weshek has mentioned The Solo Character is the character with the most scary or terrifying look, owner of a not so friendly smile. It also is red and almond white when JOLLY 4 happens, so that red color may most likely be blood and visible organs.

Also, as said before, the solo was once seen in TV, so I believe it was some kind of report about the collapse of the factory and the found corpse of Jack.

So... that would be it! I may have forgotten any other details that would reinforce my theory, but it's 2am and my body is begging me to sleep. I made sure to mention the strongest points tho!

If I'm right, remember me when JOLLY 4 comes out!

If I'm wrong. remember the fool who thought The Solo Character was a walking corpse!

I have nothing more to say, I hope I have developed a good and probably correct theory, I would be more than happy to read your comment on this! either positive or negative, both ways could contribute to my theory.

I hope you enjoyed reading, and don't forget to Stay the Course!

Bye!