Willy is a special weapons trader around Modoc. It uses a special method which is going to you sleep and then you propose a weapon when you don't know what it is. Among the weapons it provides, there is the rare Alien Blaster. This is the one way to have it in Fallout 2.
The next information is taken from "The Nearly Ultimate Fallout 2 Guide" by Per Jorner:
A trader and guards: Go to the top of the map directly north of Modoc and walk around that block and the area extending roughly two squares in all directions. Eventually you'll get this encounter. Turns out there's a trader named Willy who wants to sell something. Problem is, while talking to him you fall asleep, and must then choose if you want to buy a weapon without knowing exactly what it is. The solution? Use Steal on him before talking to him, or just hit the barter button in dialogue, and you'll see what the item is (he'll have 1-5 of them). What weapons he might be carrying depends on your level:
1-10: Sharpened Pole
6-10: 10mm SMG
10+: Power Fist
13+: Alien Blaster
The script picks one of these weapons at random and then checks to see if you match the level requirement. If the Alien Blaster was picked you must also pass a Luck check or the result is re-rolled. The price Willy asks in dialogue is the weapon's usual barter value (excluding ammo) multiplied by (11-LK), meaning he'll ask $10,000 for the Blaster if you have Luck 10. You may be able to get a better price just by hitting barter. As an additional curiosity his male guards all carry 9mm Mauser, a rare weapon. This encounter will happen any number of times, and you can kill Willy and his guards and still get the same encounter again.
Appearances in games
Willy appears only in Fallout 2.