While a minor character in the Back to the Future trilogy, Linda McFly is an essential component of the world-building. She provides a tangible example of the "ripple effect" caused by time travel; her transformation from a fast-food worker in a dysfunctional home to a happy, confident woman in a prosperous family visually confirms the success of the protagonist's mission. Her character validates that the changes made in the past benefited the entire McFly lineage, not just the protagonist.
In the original 1985 film, Linda McFly (portrayed by Wendie Jo Sperber) is introduced as the middle child of George and Lorraine McFly. Born in 1966, Linda initially represents the mundane, somewhat stagnant life of the McFly family in the "original" 1985 timeline. She is often seen at the kitchen table, dealing with the relatable but unglamorous struggles of early adulthood, such as job hunting and navigating a lackluster social life. Her character serves as a baseline for the audience; she is part of a family that is functional but lacks the confidence and success that Marty eventually helps them achieve.
Linda McFly acts primarily as a supporting character whose presence helps establish the domestic atmosphere of the McFly household. She does not drive the main plot but provides context regarding the family's dynamics.
