While sitting on this theory for some time and thinking, i have come to think the pirate lord and mysterious stranger arnt just business partners, but secretly brothers!
This thought came to mind when i realized the only thing we know about the stranger is that he is highly trusted by the pirate lord, but there doesnt seem to be any explanation why. So the theory i have is that the pirate lord and mysterious stranger is working for his brother out of respect for his brothers passion, but the 2 keep their relationship secret as to protect each other so their enemies wont know that they would be a vulnerability to each other. Why else would such a serious, gloomy, almost stoic character be the trusted emissary of the pirate lords otherwise cheery bright crew?
The other thing that brings me to think it even more is they have a similar look to them. They both have the same nose, and broad cheek bones, and what little i have seen of the pirate lord when he was living, he is tan like the stranger.
Im very curious what thoughts anyone has on my wild theory, and if anyone has any other connections that can be made, possibly from the books if there is anything in them that could add to the theory, and if you disagree, let me know why!