To unlock the Hermes's Homie trophy in Assassin's Creed Odyssey, you need to "unveil" all sub-regions of Greece. That is, you need to defog the whole map.
There are two types of de-fog; land and sea. Confirmed:
There is one section of land that is not labelled as a named sub-region, and none of the sea is labelled, but you do
have to uncover these in order to unlock the trophy.
Land looks a muddy brown and indistinct until you uncover it, when detailed features and colour start to appear. Sea is covered in ancient cartography style paintings until you visit the area.
When you are looking at the map, zoom in twice so that you can see the white edging around distinct sub-regions (as opposed to the blue or red edging around larger regions at higher zoom levels). You need to make sure that every single one of these white-edged sub-regions and the sea regions are uncovered.
Unfortunately it's a little indistinct as to when the uncovering occurs. You certainly don't need to unfog the entire area manually -- the whole area should fill in after a little exploring. However it's certainly not enough to simply go past the area. I would recommend visiting at least one location within the area (if there's one within a reasonable distance) and check the map to see if it's now showing uncovered.
Sea regions are a little easier -- pretty much as soon as you touch the edge of a region the whole section will fill in.
Here are some trouble spots, areas that people tend to miss:
Valley of the Kings: Likely a glitch, but this area between Pass of the Warriors and Limnai Marsh in Lakonia (bordered on the north side by the Smuggler's Forest region of Arkadia) is the only land-based sub-region that doesn't have a label. Here's a screenshot:
Kradia Isle: Most little islands just off the coast of larger land masses are part of a sub-region on the larger mass, but in some cases even the tiniest of islands is its own sub-region. One example is Kradia Isle south of Samos:
Chytra Atoll: another small isle that is its own sub-region south of Kythera Island:
Otherwise it's really just a case of methodically going through each brown area and making sure it's actually de-fogged; some areas are almost as brown uncovered as they were in their de-fog state so just look for small details that reveal that the area is already un-fogged. The best example of this is probably the Volcanic Islands in the south-east; Nisyros looks as muddy and indistinct as ever, yet when you zoom in you can see lava and structure detail which means it has been visited:
As soon as you de-fog the last area of land or sea on the map, the Hermes's Homie trophy will unlock.