The Harvest Goddess is an important character in Friends of Mineral Town but is not someone you will actually meet unless you have a curious spirit. To meet her, you have to find her, which is not obvious for new players. To find the Harvest Goddess, head to the Hot Springs and Spring Cave area of the map.
Right beside this is a river, and if you stand close to the river with an item in your hand, you can opt to throw it into the water. Doing so will summon the Harvest Goddess, and she will float above the water and speak with you.
Depending on the gift you give her, she will either like it or dislike it. If you want to make sure you are giving the gifts that she likes, pick plenty of flowers as she loves them. She especially likes red and blue flowers, and will always react positively when you give them to her.
It is good to visit her for 10 days in a row and give her an offering each time. On day 10 of the offerings, she will give you a Power Berry. When you get it, your character will automatically eat it, and if you look closely, you will notice that they gain a new slot on their stamina bar, allowing them to do more work before they become fatigued. For the rest of the items you can get from the Harvest Goddess, be sure to check out our useful guide.