It's all thanks to a custom Sega Genesis cartridge ... Each subsequent Sonic film has introduced a new character, with Tails and Knuckles appearing in 2, Shadow in 3, and a mysterious fourth ...

Jim Carrey delivers double the Eggman, playing Dr. Ivo Robotnik and his grandfather, Gerald Robotnik. The elder Robotnik debuted in “Sonic Adventure 2” with his granddaughter, Maria.