From the Super Mario Wiki
Oaklie is a bizarre, small, leaf covered and bearded inhabitant of Forever Forest. Not much is known about this mysterious, enigmatic character, though he appears to act as a guard in Forever Forest, instructing those who have business in it through the place, and sending those who shouldn't be there to the nearby Toad Town.
In Paper Mario, Oaklie appears to Mario after a he travels through Forever Forest a bit and warns him not to go any further, while blocking one of the forest gates. When Mario tells Oaklie that he has business at Boo's Mansion, Oaklie will let him pass, though not before giving some instructions to him about keeping his eyes open in the forest. After giving Mario this advice, Oaklie will hop off, going deeper into Forever Forest. Later, in another part of Forever Forest, Oaklie can once again be found in a small clearing. Like before, Oaklie will tell Mario to keep his eyes open and to watch his surroundings. He is never seen anywhere else in the forest.
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