Although this isn't a theory I myself belief; it is one I was requested to do, and so I shall do so to my utmost ability.

To start the theory off, we should establish the base; Andrew is a member of the mci, and their were actually 6 missing children.

When he died, he possessed golden freddy or some other suit, possibly with a green crocodile mask.

Into the pit has 6 children being killed by pittrap(a rather harmful fellow), which at that point in the books and games alike, charlie would not have been killed or even at the restaurant. Which indicates that this 6th person is their own entity. Into the pit is clearly meant to establish when the mci did take place, so it wouldn't be too far a stretch to assume that a kid killed in that mci also exists in the games.

Andrew has a crocodile mask, as noted by literally every andrewmci believer, but what's also noticeable on how we know another green lizard in the series who it could also represent: Monty.

Yes, alligators and crocodiles are not the same thing, but at the same time; we were told it was a crocodile from someone VIEWING the mask, and not the one wearing the mask.

So the viewer could have mistaken the two, its a extremely common thing where people can't tell the difference between a alligator and a crocodile.

And it goes even further, monty could have seriously been a crocodile but just changed overtime because people considered him a alligator. If the shoe fits; wear it. And Fazbear entertainment loves money.

Monty is extremely violent and destroys everything around him, like Andrew does,

Hes persistent down to the last arm in sb after being scrapped. Like how Andrews persistent in torturing afton.

So is it too much of a stretch to assume he's connected to monty and in the games..., as well as the happiest day which he shares a mask with?

Perhaps. Lets move on.

Something to note is that toy chica in the anime toy chica high school cut scenes in ucn does kill 7 People.

Excluding puppet. Its 6. Just like Andrew mci.

The list being in total, Foxy, Freddy, Wolf, Toy Bonnie, the Mangle, the Puppet, and Pigpatch.

Now, if we just remove the puppet, it just leaves freddy, foxy, wolf, toy bonnie, mangle, and pigpatch. On which each could connected to a certain animatronic, (Freddy-gabriel, toy bonnie-jeremy, etc)

But of this list, one seems off, no?

On this list, every single person has made some kind of appeareance in game, some evidence of their existence.

Except wolf. Which has only been in the twisted ones.

And just because Roxanne is also a wolf doesn't mean their THE wolf, its a very clear different design.

One animatronic who had never shown up in the games matching with the one child who had never been mentioned in the games; almost a match made in heaven regarding evidence there.

In the man in room 1280, he clearly is recreating the conditions that are ucn; a eternal hell in aftons mind because of the rage felt against william. Which matches Toysnhk to a tee regarding rageful behavior. and would make sense on why old man consequences is telling Toysnhk to rest; we see how rageful Andrew is and how much that comment would be warranted. Andrew wished to torture afton forever for what he did to andrew, just like Toysnhk.

Toysnhk is also confirmed to be a male which, unlike the unconfirmed cassidy, andrew is confirmed to be a male and fit the "He" they refer to.

And then, theirs the fact that you have to pick up 6 balloons in Pick Up The Balloons in ITP , while your character has a red shirt and black hair. It being next to golden freddy and killed by fetch in the minigame, both of which hes connected to, heavily indicates this is referencing Andrew.

Which adds another layer to it when you consider the fact that their were 6 balloons he picked up, but only 5 when fetch kills you in the mini game; because the 6th balloon was for you.

@ScraptrapOrCaine hopefully this satisfies you. I tried looking for evidence but it was hard to find concrete leads; so I just went with confirmed leads among the evidence.