Grandfather is the main antagonist of Operation: Z.E.R.O.. He is a demonic being who ruled the world before he was overthrown by his son, Monty Uno. Many years later, he was revived by his son and successor, Father and began transforming the people of the world into Senior Citi-zombies. Grandfather is very tall and lanky, and significantly taller than a normal person. He had a bald head and a large beard with long, scraggly strands of hair. Despite making only one appearance, Grandfather is by far the most powerful and evil villain and also Bigger Bad in the series and for all Cartoon Network.