The Duke is a member of the Village, a small but close-knit community based in Eastern Europe. He runs his own merchant business named The Duke’s Emporium in which he provides medicine, weapons, ammunition and upgrades. He will also purchase certain items and it other valuables. Later in the game he is able to cook special recipes which boost a particular skill of Ethan's permanently, in exchange for the right ingredients and some Lei for good measure. It's assumed that Duke and Ethan both eat the meal at the same time, although this is not seen in game, only audio is heard.
He travels around in a customised wagon and appears to know the Village extremely well as he always seems to be in the right place at the right time (although this is more of a game mechanic choice rather than something which can be explained) - in fact it is not certain that he is a human, he could very well be a mutant of some kind as Ethan once asked him "what are you?", and he could not give a straight answer.
He first meets Ethan Winters
at the entrance of Castle Dimitrescu in which he provides his goods and services to him in exchange for the local currency known as Lei. He can then be found in all major areas of the game, and on one occasion he rescues Ethan from his doom.
Although he mainly serves to assist Ethan on his quest, he does not choose sides - he will do business with anyone providing they can pay, as he has been known to trade with the inhabitants of Castle Dimitrescu and the Village as a whole.
Interestingly The Duke seems to be aware of the Ganado
Merchant (The Merchant
) who was featured in Resident Evil 4
. He even specifically quotes him, saying “What are ya buyin’?” and then proceeds to laugh and tells Ethan it was what an old friend of his used to say.