Mudslinger is an old villain who was hired by Brit and Tiff to ruin Jenny's new paint-job on the night of Don Prima's party. He was once part of "The Legion of Evil." Judging by his name, he was once a journalist for a newspaper, but he was presumably fired for writing untruthful articles.
Brit and Tiff were having a party with Don Prima and were upset by the attention XJ9 was receiving (on account of a new paint job and upgrade). Resolving to be the centre of attention as they deserved, they hired the mudslinger to destroy Jennys new look and literally "muddy" her up. He succeeded in this, but Jenny was so enraged that she nearly destroyed him, he confesses that it was the Crust Cousins' idea which made it another reason why she should destroy him. The Mudslinger broke down, saying that he had never even been invited to a party and knew how she felt. Jenny, in response, kindly offered him her invitation, on the condition that he got back at the two who dirtied her up. The Mudslinger made good on the promise, using his pen to stain everyone at the party and proclaimed that the cousins invited him, destroying their reputation with the popular people there.