From Nookipedia, the Animal Crossing wiki
Joan (カブリバ, Kaburiba) is a special visitor who comes to the player's town each Sunday morning (6 AM to 12 PM) to sell turnips. She is a brown boar with large white tusks carrying a basket on her back with white turnips inside. She wears ragged blue clothes and a small, blue bandana on her head. In Animal Crossing, Joan would only sell one type of turnip, however, in Animal Crossing: Wild World and Animal Crossing: City Folk, she sells two types of turnips
Joan's background is unknown, but she appears in town before 12 noon in every Animal Crossing series so far. In Animal Crossing, she sells white turnips. White turnips, however, sell for a different price depending on Stalk Market. The Stalk Market is a play on the existent Stock Market in reality. More than one can be bought a week from Joan. Players can ask Tom Nook what white turnips are currently selling for at two different times of the day, which will have different prices and he will buy white turnips between Monday and Saturday.
In Animal Crossing: Wild World and Animal Crossing: City Folk, she continues to sell white turnips which work the same way as they did in the original games. She now also sells red turnip seeds for 1,000 Bells which will need to be planted like flowers, and be watered each day so they can grow to their full potential and later sold to Tom Nook for profit. Only one red turnip can be sold to player(s) each week.
Joan can be asked to explain her store and turnips, but she wishes not to explain how her store works. However, she does not act gloomy while explaining.
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