Miss Wren created two loops, her Menagerie and another unknown loop.
Miss Wren is mentioned frequently in the beginning of the story, but only appears near the end.
Because Miss Wren was the only ymbryne to escape Wight capture, the peculiars who escaped Miss Peregrine's island went to search for her in the hope that she could help 'Miss Peregrine' out of her bird form.
At the menagerie, Addison shows the peculiars a picture of Ms. Wren.
When the group reached the loop at the London Underground, they looked out for peculiars and found Sergei Andropov. They asked him if he knew Miss Wren, but he dismissed them. After walking out of the sideshow, a boy told them to go to Wakeling and Rookery. Curiously, they follow the boy's instructions and found the building where the Council of Ymbrynes convenes and Peculiar Archives residing at the intersection, completely covered in ice. They kept on finding a way to enter the building, until Miss Wren stepped out of the building. They followed her desperately and had a talk with the ymbryne. After learning their cause, she immediately called Althea to prepare her equipment.
Miss Wren worked on Miss Peregrine for hours and was shocked when she learned that Caul was the one she had been helping. She confronted Caul, but he killed Althea which caused the building to unfreeze, thus, letting the wights and hollowgasts come in. Miss Wren, thinking of the peculiars' lives, told them to go along peacefully. They were captured and went to the present time at the train station. Before the train roared away, Jacob saw Miss Wren being shoved into the train car.
Library of Souls
The snapshot that is found by Emma shows Miss Wren wearing a black sequined shawl and her hair in an upsweep. By the time the group of peculiars finally meets her, her hair is completely silver, with no trace of a black highlight.
She is an ymbryne, which gives her the power to manipulate time and turn into a bird, specifically a Wren.