Robert De Niro's character in "Little Fockers" has the middle name Tiberius: he is Jack Tiberius Byrnes. The character is Pam Byrnes's father and Greg Focker's intimidating father-in-law.
Jack's full name is a small but memorable detail from the "Meet the Parents" film series. His formal, almost Roman-sounding middle name contrasts with the character's tightly controlled personality and helps distinguish him from the many other family members introduced across the films.
Jack Byrnes first appeared in "Meet the Parents" (2000), returned in "Meet the Fockers" (2004), and appeared again in "Little Fockers" (2010). Robert De Niro plays him as a retired CIA operative whose suspicious, surveillance-minded behavior drives much of the comedy.
A common mix-up is confusing Jack's name with Greg Focker's. Greg's full name is Gaylord Myron Focker, while Jack's is Jack Tiberius Byrnes. The middle name Tiberius belongs specifically to De Niro's character.