Robert is a boy who was once Greg Heffley's classmate in The Meltdown. Greg states that when the class had a long-term substitute for the year before, he and all of his classmates switched seats so that the sub would refer to all of them by the wrong names, with Robert switching with an unnamed boy.
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The substitute calls the unnamed boy sitting in the front Robert, but as you can see in the back, the actual Robert is sitting behind the unnamed boy. So the substitute is referring to the classmates by the wrong name.