The girl from our world who travels with Eustace in The Silver Chair is Jill Pole.
Jill is Eustace’s classmate at Experiment House, the English boarding school where the story begins. After Jill is bullied, Eustace tells her about Narnia and suggests asking Aslan for help. Their escape leads them through Aslan’s Country, where Aslan sends both children into Narnia to find the missing Prince Rilian.
Jill receives four signs that are meant to guide the quest. Remembering them becomes one of her main responsibilities, but distractions and danger cause the travelers to lose their way. She journeys with Eustace and Puddleglum through the giant lands and the Underland, eventually helping expose the enchantment controlling Rilian.
Jill is not Lucy Pevensie, Aravis Tarkheena, or Polly Plummer. Lucy belongs to the Pevensie family and appears in other Narnia adventures; Aravis is a Calormene noblewoman from The Horse and His Boy; and Polly is Digory Kirke’s companion in The Magician’s Nephew. Jill later returns in The Last Battle.