Even though the Lemon is used for cooking and warding off the Anti-Guy, those things are only optional. The mandatory purpose of the Lemon is to give to a green nomadimouse. Then, the mouse will tell you to buy a Dried Shroom and then a Dusty Hammer in that order. If you do, you can find Moustafa, who will give the Pulse Stone to you. Because the Pulse Stone is an important item that must be gotten, finding a lemon is required.