He finds and eats the pot-roast which contains Kenny's soul when Chef, Eric Cartman and his mom accidentally leave it at the airport in Denver. In a spoof of the plot of many of his films, he is transformed into Kenny McCormick and eventually dies as Kenny, getting shot in the head and impaled on a flagpole, mirroring Kenny's death in "Weight Gain 4000". The resulting movie was entitled Kenny.
Rob was other different things before he was Kenny. He was a stapler in the first movie, then a carrot in the second movie, and then a Da Derp De Derp Da Teetley Derpee Derpee Dumb in the third. He then had Kenny's soul in the last.
He is later revived somehow and seen among the celebrities threatening to sue South Park in the promotional image for the episode "200". He doesn't actually appear in the episode but he does appear in "201". He appears in his carrot form.
He reappeared in "Funnybot" in a silent role as one of the comedians attacking South Park Elementary. Unlike other celebrity characters from the earlier half of the series, his design has not been tweaked.
Over at the cinema, several movie trailers for Rob Schneider's movies including some listed above can be heard in the movie theater.