The character of Trudi Montag is described as having genetic dwarfism, a condition that causes the body to stop growing in height at a certain age while other parts of the body, such as the hands, grow larger in proportion. Because the story concerns themes of loneliness and isolation from community, Trudi's condition allows her to occupy the role of "outsider protagonist," a central character who is not fully part of the society described in the book. This also allows Trudi to be more sympathetic as she experiences the rise of Nazi Germany and the Aryan push for genetic perfection.
As a child Trudi Montag thought everyone knew what went on inside others. That was before she understood the power of being different. The agony of being different.
(Hegi, Stones from the River, amazon.com)
Trudi is first isolated from society because of her condition, and so better understands the plight of Jews and Gypsies, as well as anyone deemed "undesirable," during the War. Her position allows her to gather secrets and act as an ally for those the Nazis would have killed; she is usually ignored because of her perceived inferiority. The exclusion Trudi experiences gives her the understanding to help others, and to affect people and events that are not directly connected to herself. It also shows how her physical characteristics are taken as stereotypical by others; instead of understanding that her body does not affect her mind, she is assumed to be mentally inferior, and that causes people to underestimate her.