Gertrude Tannen lived with Biff at their house in Hill Valley. It is likely that she is the mother of both Irving "Kid" Tannen and Frank Tannen. She was never shown on-screen, but her voice was provided by Thomas Wilson, notably where she is yelling at her grandson Biff (also played by Wilson).
Gertrude was known to be an extremely unpleasant woman. This is supported by the fact that on his house's fence had two notices; one of them said: "No tresspassing! Violators will be prosecuted. This means YOU!", the another notice said: "Keep off the grass". Thomas Wilson states in an interview that the incessant nagging from Biff's grandmother may be the primary reason Biff grew up to be so mean. She could be heard yelling to Biff the night of the Enchantment Under the Sea dance.
- Back to the Future Part II (voice heard only)