Travelling Merchant is a special character in Dave the Diver that was added on with the free December 15 DREDGE DLC content update.
After the DLC content is installed, she'll appear on the first thick fog night in-game to tell Dave about the aberrations that appear on such nights. She'll then provide a special light, Sky Fire, that Dave can use on his ship to navigate on thick, foggy nights. She'll also subsequently buy any leftover aberrant fish that haven't been used by the restaurant as the fish can't be stored like the rest.