After you get Thernite, go to the forest. Go to where the guardian trees are found. Nearby there should be someone who
has ?! above his head. Talk to him. He wants you to get ride of the boulder in front of the entrance to his house. Use Thermite to blow the boulder up with one of his bombs. Then, collect your elemental fruit and go into the house. Open the closet and you will find Thermite's alternate skin.