A similar character to Sad Sack, T-Shirt Man basically fulfilled exactly the same patron role—that of a quiet, loner, oddball, given to behaving himself at all time—albeit wearing a white T-shirt, year round.
For a long time I suspected that T-Shirt Man chose to wear only white T-shirts because he couldn't afford any other kind of shirt. However, in this day and age, t-shirts come in a wide variety of styles and colors, all for pretty much the same price. This guy could choose a different color for every day, but he sticks with white. This has lead me to theorize that T-Shirt Man, much like his fellow patron, Fatty Manchild, must have spent a goodly number of years in a coma following a period of arrested fashion development.
T-Shirt Man's fashion arrest may have come after viewing the classic biker gang flick, The Wild One. In this theory, T-Shirt Man went to see The Wild One and really thought the look sported by the young Marlon Brando—white ringer t-shirt stretched over his frame, biker jacket, hat, angst in eyes—was super keen. Unable to find himself a proper ringer T-shirt, biker jacket or hat, though, T-Shirt Man settled for plain ol' white and began sporting the look as best he could manage. Then, following a tragic 15 mph moped accident, T-Shirt Man was sent into a coma and awoke 30 years later to find himself in a world he hadn't made, fashion sense forever frozen that moment in his youth. And his vanilla, white-clad, "Mild" One saga continues to this day during his rare visits to the "liberry."
(T-Shirt man also disinguishes himself from Sad Sack by his ability to use a computer, no doubt prowling ebay for bulk deals on more of his namesake.)