In the morning or afternoon hours, you'll find a sickle-wielding weasel hanging around the waterfall. Weasel thinks flowers are very important, and since you do as well he asks if you would help cut some weeds with him.
You will get the opportunity to play the Weed Cutting mini game with him. The score you receive won't matter, as the fact that you're willing to help him remove the pesky plants is what matters to him. Once you finish with the game, the weasel will offer to help you with anything you need.
Walk back to the Waterfall area and head to the northeast corner. You'll find a bunch of purple, wild growing weeds. The weasel might be able to help you clear the path, so investigate the weeds (press A when standing next to them) and then head back to the weasel.
Return to talk to the weasel and ask for his assistance. He is willing to cut the weeds down with his sickle, but he requests that you bring him 5 Char first. You can find the fish in the Mountain Stream, but they are more plentiful right here in the Upper Waterfall area.
Bring the 5 Char back to Weasel and he'll get ready to go to the Waterfall, but only after complaining that you took too long.
Back at the Waterfall, Weasel will cut down the purple mess of plants for you. Now you can move onto the next area.