Rupert is an imaginary character created by Greg at the end of Diary of an Awesome Friendly Kid as a "best friend" of Greg, intended to replace Rowley's role in Greg's diary. Greg threatens to do this if Rowley gets too greedy and tries to cash in on his fame so that Rowley wouldn't be named or credited. Rupert is a parody of Rowley but exceptionally stupid and always drools. He is imagined as a dumb drooling kid with messy hair and an R on his shirt.
Prior to Diary of an Awesome Friendly Kid, Greg considers mixing up Rowley's identity to Rupert or a similar name in his future autobiography Greg Heffley: A Life to make it so Rowley seems insignificant to Greg's life.
However, in the German translation of the Diary of a Wimpy Kid series, Rowley is renamed to Rupert Jefferson. It is unknown why this change was made.