The narrator is possibly a past friend that he was close with who knows a lot about Drake. Obviously something went wrong and they're no longer friends. Perhaps they were looking for whatever the teaser alludes to with all the pirate references. Hence everything we see is actually their own research stuff.
In UC3 we hear a quote and see a bunch of research stuff too.. so why not?
Drake might've decided to take the gold for himself or just prevent anyone from getting it due to his moral compass. Or maybe he just discovered something and one thing led to another and he had to betray the other guy.
Now this other guy clearly knows a lot about Drake.
Some chains can NEVER be broken. -> They are connected.
I lost 15 years burried alive, erased. You left me. Rotten, in that hell hole and never looked back. -> Feeling of betrayal.
But you cannot outrun the past. -> They go way back.
And when it catches up, when all your lies collapse around you. -> He's obviously mad and knows Drakes secret.
Ill be there. Sifting through the wreckage. You owe me.
I'm calling it.
The narrator is Drake's brother. Sully might know something about it or maybe Drake kept him a secret like so much else.
