We learn quite a bit about the physical descriptions of many of the characters throughout Jeanne DuPrau's The City of Ember. We learn many character descriptions in the opening chapter and others throughout other chapters.
In the opening chapter, we learn Lina Mayfleet, the first protagonist, is thin, has long dark hair and is dressed a bit raggedly because the storerooms are so low in stock on clothing. Lina is dressed in a "ragged cape" and socks that are "loose and tended to slide down around her ankles." Later, in chapter 2, when she begins working as a messenger, her ragged cape is replaced by the red jacket that is part of the messenger uniform. We also learn she has long legs she loves to run with, and she thinks she is plain except when she's very happy.
In the first chapter, we learn that Doon Harrow, the second protagonist, has rumpled hair and "dark, thick eyebrows" that made one straight line depending on his emotions. He is also dressed in an old and tattered corduroy jacket.
In multiple chapters, Lizzie Bisco, Lina's best friend, is described as freckle-faced with bright orange hair.
In the second chapter, we learn Poppy, Lina's two-year-old sister, has big brown eyes and brown hair, just like Lina.