The Silk Seller is a minor character in Chapter 3. She has a stand in the market and provides Fran with the string she needs to finish the chapter. The string is used to connect the fishing rod to the hook, and catch fish in the river to complete the list of riddles from the Great Wizard.
The silk seller seems very kind and generous, as she gives Fran the string for free. She also seems a bit clueless, as she admits she has never sold anything before and doesn't know how much she should charge.