Not game related, but I have always like the wandering shops. https://wiki.lspace.org/Wandering_Shop

Only by reading this thread's title comes into my mind the mystery trader of Stars! 2.0

What do you make of the Town Tool Shopkeeper in Mabe Village from Link's Awakening? The one who renames your save file name to "THIEF" if you steal from him and instantly kills Link if he returns.

The Mystery Trader from Stars! A wandering, fast flying ship that zoops though the galactic map every so often, broadcasting a request for materials and offering prizes in return.

Oops. Same wavelength as Lone_Scout.

Pretty cool, never heard of it before!
He is cool, but if I remember correctly, he is a one-location merchant. I was thinking more on those that can be found around the world map, always with zero explanations, just because they can.
I have never heard of this game, but the type of merchant seems to be right on the spot. I will check out more information about the game, thanks to you both for suggesting it!

I own Darkwood, have not played it yet. Does this dude shows up right at the first couple of hours?

tHey show up really late in the game, so if you do play Undertale don't expect him to take up the screen.
https://www.gog.com/en/game/undertale

Indeed, you are correct. Then the merchant from Chained Echos should fit the bill. Close to every boss fight there is always a merchant to purchase from and an anvil to forge equipment on, following the player until the very end of the game - admitting to doing so as it is good for business. Honourable mention to the Save Frogs from Mother 3. Not a merchant, but they always appear right when they are needed most so you can save the game.

Yes, see screenshot attached.

✅ covered in capital-M Mysteries
✅ can be found around the entire gaming map
✅ intentions or affiliations are usually unknown
✅ don't necessarily like you, just your cash
✅ real character with cool visuals

N1.

ah, the only other one I can come up with is the Gun Runner's Arsenal robot, that I DID NOT know was named Vendotron. He's impossible to kill or even pickpocket as they are elbow deep in protective glass. No in. No out. Just a permanently functioning robot to buy guns from.

It's the one who shows up every morning at your house.

Hummingway merchant in Final Fantasy 4.

There's a cave, on the moon, which is filled with a bunch of Hummingways, all of which look exactly like Namingways. Most just hum when you talk to them, but one of them will open the shop menu, allowing you to buy consumable items including Elixirs and Whistles. (Be aware that Elixirs are *expensive*!)

There's also an actual Namingway living among the Hummingways. (In 2D versions, he offers to rename your characters.)

Some others in the Final Fantasy series:
* FF1: In Lufenia, there's a hidden merchant, outside of what appears to be the bounds of the town, who sells Arise and Flare (LIF2 and NUKE in NES translation); I believe you can even buy them even if you haven't yet learned the language.
* FF2: The final dungeon has a merchant hidden behind the waterfall who sells powerful spells.
* FF3: At the end of Eureka, there's a merchant who sells 8th level spells, one who sells powerful summons (in 2D versions, only if you've earned them first), and a hidden merchant who sells crystal equipment and shurikens.
* FF5: There's a mysterious town, entered in what at first looks like a random encounter (in the SFC version; later versions spoil this by using a different transition for random encounters, but not for the town entry), with merchants who sell either high-end or unusual equipment.

Dragon Quest games have their share of unusual merchants, as well:
* DQ2: One of the merchants has a blank slot on the list of items for sale.
* DQ3: There's that ghost town, which is inhabited only at night. My favorite thing isn't the merchant, but rather the innkeeper; you pay for a night at the inn, go to bed, and there's no innkeeper there when you wake up.
* DQ4: That one fox merchant who lies to you. When you buy an item from the list, that's not the item that actually enters your inventory. This merchant appears in Chapter 3, and isn't the only unusual one in the chapter. There's one who you can work for, and if an item is sold by an NPC, you can buy it back yourself (including one particular sword that's rather nice for this point in the game). There's random encounter merchants. (You get into an encounter, but instead of a fight, you get to shop.) And, of course, your character is a merchant in this chapter (though I suppose that one doesn't meet your criteria).
( DQ7: There's that one bird merchant. Shopping with that merchant is like navigating the shop in a JRPG in a language you don't understand. There's also Tarhiel. He doesn't have anything for sale, has no money (so you can't actually sell him anything), and actually talking with him is not so straightforward, particularly since he spawns in mid-air and doesn't have enough health to survive the landing.

Think there may be some merchants in the Elder Scrolls series who start out hostile. (In other words, they'll attack you as soon as they see you.) I believe Morrowind's master Enchant trainer is one of them.

I would need to double-check, but I’m quite sure there is one in Discworld II: Missing Presumed...!?.

Yeah my first thought was the Merchant from RE4 (great quotes, will stop appearing in a specific location if you kill him in that location on Professional difficulty, and doesnt sell if there are enemies that occurs in one location). The mystery comes from the fact that the merchant himself is also infected by las plagas but is not mind controlled by the big bag.

Goes into fan theory but the shop keeper in Slay the Spire. The game itself is quite mysterious (your character is tasked with slaying the spire but we dont know why asaik but the "cards" are implied to be memories/a UI for the player to choose what moves the character will do in battle. The merchant can sell these cards which means the merchant can arguably be teaching you these moves.