*Marcy is Velma Dinkley's cousin and the daughter of Evan and Meg.

Marcy flirted with Fred Jones on sight, which made Daphne Blake jealous. To make matters worse, she was also catty towards Daphne; when Daphne said she saw something terrible in the window, Marcy suggested she just saw her own reflection. She also rudely called Daphne, "Daisy" and then "Debbie". Despite this, she was still happy to see her cousin, Velma as they hugged each other when the latter arrived.

She was born on Halloween and grew to resent the holiday because it always upstaged her birthday every year. She built the Scarecrow Robots and created the holographic projection of the Ghost of Hank Banning to ruin Halloween for the others, which was why she took an engineering class in school.*