In television
In Twin Peaks, Maddy first appears midway through the first season, when she travels to Twin Peaks from her hometown of Missoula, Montana (David Lynch's birthplace). She comes to comfort her uncle and aunt, Leland (Ray Wise) and Sarah Palmer (Grace Zabriskie), after the death of their daughter Laura. Maddy is four years older than Laura, but otherwise looks identical apart from her dark hair and glasses. She remarks that she and Laura used to pretend they were sisters. Despite their resemblance, the innocent and sweet Maddy stands in stark contrast to Laura, whose personal life is steeped in deception. Like Laura's mother and Laura herself, Maddy has premonitions, including one of a bloodstain on the floor of the Palmers' living room and another of BOB (Frank Silva), a demonic entity plaguing the town. Maddy quickly befriends Donna Hayward and James Hurley, Laura's closest friends, and helps them in their investigation into Laura's death. At one point, Maddy even wears a blonde wig to lure one of Laura's acquaintances.
During the second season, Maddy begins to resemble her cousin more and more: her hair (though still dark) straightens, she stops wearing her glasses, and in her carriage and demeanor she behaves more like Laura as seen in Twin Peaks: Fire Walk with Me than the Maddy of the first season. This causes conflict with Donna and James when James becomes attracted to her and she begins to return his feelings in spite of herself. Later, she is murdered by Leland Palmer, who is possessed by BOB, in a violent recreation of Laura's murder. Maddy's death quickly leads to Leland's arrest.
A doppelgänger of Maddy later appears to Dale Cooper (Kyle MacLachlan) in the Black Lodge, warning him to "watch out for my cousin".