Amanda Hollister is the younger sister of Mackenzie and not much is known about her.
She was briefly mentioned in the first book after Brianna kicks Mackenzie and Mackenzie thinks it was Amanda. Her first appearance, however was in the third book at the Queasy Cheesy with herself, Mackenzie and Jessica.
She seems to have a very similar personality to that of Brianna Maxwell. The two once interacted with each other near the beginning of the third book, where Amanda thought Brianna was a real pop star and even asked for her autograph. She also believed that Brianna had a pet unicorn that she was going to buy from the market. They seem to have known each other prior to this meeting, though.
She has brown hair with bangs and two barrettes. She appears to be the same age as Brianna Maxwell.