We have next to no information on who the protagonist for Evergreen could be. When you think about it too, it seems like there aren't really that many options for who it could be anyway. Despite Kane's vagueness on them, Strings could be making a return with a new story behind them. On the other hand, we could be playing as Fritz himself. But then I thought about it some more, and came up with another; the Fazbear's Fright nightguard.

Fritz travelled to America to attend the auction after the Fright Fire. It's possible that while there, he ended up meeting the night guard for the attraction. One thing could lead to another, and the night guard ends up moving to England and getting a job at Popgoes' Pizzeria.

But why would the night guard travel, let alone move, to an entirely different country on a whim like that? To be frank, I don't have an answer for that. Like I said in the beginning, we have basically no info on who the protagonist might be. But, that's just it, we also don't know anything about the FNaF 3 night guard, not even their own name! And to add on to that, Kane himself during a QnA here answered a question about this nightguard.

He says that the protagonist of FNaF 3 is "a mystery for now". In the original POPGOES series/canon, the FNaF 3 night guard has next to no importance in the story whatsoever. If that were the case still for Evergreen, why would Kane say this? Evergreen has a different story to the original game, and one of those changes could involve the FNaF 3 protagonist.

I could obviously be off the mark here about them being Evergreen's protagonist. But, it's possible at the very least for them to still have some role in Evergreen's story, even if it's small.

TL;DR

Fritz, the owner of Popgoes' Pizzeria, visited America to attend the auction after Fazbear's Fright burned down. There, he could have met the FNaF 3 protagonist, which could lead to them moving/travelling to England and getting a job at Popgoes' Pizzeria.

The fact that Kane himself has said the FNaF 3 protagonist "is a mystery for now", seems to also imply that at the very least they might have some role in Evergreen's story, even if it's small. Which would be way more than what they got in the original POPGOES series, that of course being nothing.