Sable

I've been trying to keep track of all NPC's in the game since Sansee, though I'm sure I've missed some (especially at the Ibex Camp) so please correct me or help add to the list if I'm missing or wrong in some areas.

The fastest way to unlock this would be to travel to Eccria in the Redsee (NW region of the map) as there are over 20 NPC's there alone, the largest concentration in the game.

If an NPC is in bold letters they give you a quest.

Ibex Camp
Jadi
Driss
Hilal
Saima
Cartographer Jourdan
Sizo
Postbox (unsure if this counts as an NPC)

Cartographer's Balloon in Sansee region (2nd area of the map after leaving Ewer)
Cartographer Niaz

Burnt Oak Station - Sansee region
Scrapper Altyn
Nomadic Merchant Bobi
Guard Eliisabet
Ifaa the Innkeeper
Shepherd Lia
The Machinist Utarii

Cartographer Balloon - Badlands region
Cartographer Talon

Badlands region - 1 grid square east of the cartographer balloon
Boatmaker Jake
Harvester Umi

Eccria - Redsee region (NW area)
Maz
Garay
Serai Nomad Yuuki
Guard Sandip
Eccrine Crowd
Eccrine Lili
Hamza the Machinist
Glassblower Toto
Rita, Overhang Cafe owner
Eccrine Carlo
Doctor Val
Ha Quest
Guard Yannis
Atomic Priest Shar Varana
Bike Merchant Ahvid
Serai Merchant Josh
Merchant Iria
Pals, Jef, Nezz children
Artist Costas
Chime Maker Aeyan
Courier Joti
Fabric Seller Kalia
Tej

Thieving Magpie Well - NW of Eccria
Climber Micah

Five Bells Camp
Bashir
Merchant Habat

Cartographer Balloon - Hakoa (SW region)
Cartographer Arzu

Seven Sisters Camp - northern section of Hakoa region
Forest
Machinist Zahra
Courier Ireni
Innkeeper Afii

Crystal Plateau - far SW Hakoa
Hakoan Merchant Caimin
Crystal Farmer Tohta

Cartographer Balloon - Sodic Waste (NE region)
Cartographer Iliud

Shipyard area in the Sodic Wastes
Llhor the Fabulist
Salt Harvester Dane

Geyser Cliffs in the Sodic Wastes
Climber Dilhan
Plantweaver Rachel
Hunter Zebi

Cartographer Balloon - The Wash (SE region)
Cartographer Sifi

Marrow Bone Station
Innkeeper Ofuu
Ergert
Scrapper Jivan
Shepherd Mateo
Machinist Kaalan
Merchant Thiago
Doctor Sura
Guard Eliisabet
Sofia
Leo

Kemble's Cube
Climber Zayna

Mysterious Shrine
Machinist Fir