Charlie Tucker, more commonly referred to as "Pop-Pop" was the husband of Meemaw, father of Mary Cooper and maternal grandfather of Sheldon Cooper, George Cooper Jr. and Missy Cooper. He passed away in late 1985 when Sheldon was five years old. Sheldon missed him and wanted him back and asked Santa to bring him back, but it didn't happen and this soured Sheldon so much that he hated Christmas altogether.
He was the only member of Sheldon's family who encouraged him to pursue science and in "The Meemaw Materialization", Meemaw says that he was a stubborn and egotistical man and there were days that she wanted to fill his pockets with corn and toss him in the pigpen, but she stood by him and made him into a better man, as revealed by Sheldon.
As a result of Pop-Pop's ego and narcissism, Sheldon seems to have picked up those traits, proving that he respects Pop-Pop to the point that he takes after him.