Joshua Waits is a boy of about ten years of age, the younger brother of Holly, youngest child of Michael and Diana, and grandson of Seth. He is the main protagonist of the film. In Troll 4, he became the antagonist.
Joshua can make contact with the spirit of his deceased grandpa Seth (though the movie does not clarify how), which his parents disbelieve, and whom he misses very much. One night before bed, Seth tells Joshua the story of a young boy named Peter who was hoodwinked by a beautiful girl (really a goblin) to drink a green fluid that transformed him into a vegetable, and then eaten by her and a horde of other goblins. This story frightens him, and prompts him to ask his mother if goblins exist.
The next day, his family goes on a trip to a small country town called Nilbog, in which they are exchanging homes with a family called the Presents for a month. Along the way, he sees a vision of Seth warning the family to turn back, and when they arrive there, the Presents family’s son throws him a baseball with the words “eat before we eat you” written on it in green writing, increasing his worries.
Throughout the run of the film, Joshua constantly prevents his family from eating anything, as he knows all of the food the family encounters will turn them into vegetables. His actions range from shouting at his family to not eat party food to standing up and urinating on it. His family never does eat any food, but he never manages to convince them to turn back and go home. After Bells, the town’s preacher, is killed, the family is cornered in the house by the goblins, and Joshua finally convinces them of Seth’s spirit’s existence. They hold a séance , and he is somehow brought to Creedence’s home, and sees the Stonehenge Magic Stone there, which is what powers the goblins.
When his family arrives at the house, they all work together to defeat the goblins by grabbing hold of the magic stone and concentrating. When they return home, Joshua finds that his mother has turned into a vegetable by eating an apple, and then finds her as a vegetable and being eaten by the goblins (how they survived is never confirmed). One of the goblins, presumably Creedence, asks him “do you want some, Joshua?” and the film ends with an extended scream from him. 
In Troll 4, his mother's death took it's toll on him and he became insane.