Her loop was attacked by Hollowgasts and all of the children were slaughtered. She tried to save them, but she was outnumbered and they put the machine in her loop. She somehow managed to escape and fled to Miss Peregrine's Home For Peculiar Children. She was found by Jake and Emma in the Cairn Tunnel, hurt and unable to turn back to human form. She was brought back to Miss Peregrine and nursed back to health. When Peregrine went with Mr. Barron, Miss Avocet took charge and led the children. She was later killed by Malthus.
In the movie, she has the looks of being elderly and having white hair with streaks of black to represent the bird, she wears a gray skirt with a gray blouse. She also has a pair of spectacles.
She is an ymbryne, which gives her the power to manipulate time and turn into a bird, specifically an Avocet.
Miss Peregrine is a good friend of hers and came to her when her loop was attacked by Hollowgasts and all of her children were slaughtered.