There are several NPCs on the island.
Tik'hi Tak'he is the 'main NPC' of the island. You need to earn his trust to then be able to trade trust points with him by completing tasks.
Om'Wake Naha is the NPC who gives the Supply task to help with storing cheese. Each success is 200 points, 20 hour cooldown.
Eustacios Spy Rat is the NPC in the tunnels who needs assistance with the stealth mission and gives you this task. Each success is 200 points, 20 hour cooldown.
Captain Coohan is at the south of the island and is simply for flavour, but you do see him during the Journey as he alerts you to hazards. Each success is 200 points, 20 hour cooldown.
Captain Jack Rat captains a trusty ship which takes you to The Wreckoning which you will be able to access after trading 1200 trust points with Tik'hi Tak'he. Beware, if you take the dangerous way you may face Tentugly. Once you've taken the dangerous way once, he can take you safely anytime you wish as he discovered a new route.
There are also many Rascoohan Citizens, 6 by my count, all over the island and up in the mountains. They are only around as far as I know for roleplay/decoration and don't share any information. You ask their name and they tell you they don't share their names with strangers.
None of the NPC serve as a shop for potions or runes, however a quick shortcut to Venore on the southern dock becomes available once you begin trying to earn Tik'hi Tak'he's trust and then asking Eustacio for a Shortcut to return makes venore your best re-supplying location. Or Issavi if you are in the exotic cave spider caves.