Kipper Krupp is Benjamin Krupp's 6th grade nephew and the main antagonist in Captain Underpants and the Terrifying Re-Turn of Tippy Tinkletrousers.
Kipper and his friends Bugg, Loogie, and Finkstein like to bully the kindergarteners, especially George Beard and Harold Hutchins. Since Kipper was the nephew of Benjamin and he often calls for his uncle when he's in a jam, nobody could figure a way to give him and his friends a lasting form of punishment. George and Harold become tired of his bullying, so they decide to do prank him and his friends, such as rumoring about a ghost named Wedgie Magee haunting the whole school. Kipper starts to catch on due to finding the lock that George and Harold were using and as such changed it to a lock that couldn't be picked, however George and Harold use the locker vents for their remaining pranks. In addition to this, they sabotaged the deodorant Kipper and his friends use as well as secretly get a pizza with double ghost chili peppers, so that the bullies would unknowingly eat the pizza and nearly incinerate their own tongues. Eventually thanks to an incomplete prank by George and Harold, they were able to successfully scare Kipper and his friends out of the school (beating up Benjamin in the process by trampling him) where they were about to make a turnaround in their personalities. In the original ending, he and his friends do nice things to the kindergarteners such as carry all their books.
In the bad ending, he and his friends encounter Tippy Tinkletrousers, who had traveled back in time to the point where the bullies had just fallen for the ghost prank. Upon seeing Tippy appear before them (as they assumed he and his robo-pants were Wedgie Magee's ghost) due to it being dark, the bullies do crazy things, such as eating dirt. Kipper and his friends eventually get sent to the Piqua Valley Home for the Reality-Challenged, where the former science teacher of the Jerome Horwitz Elementary School, Morty Fyde, resides in the future. This in turn got Benjamin fired as he was beaten up and that made the authorities assume he was responsible for Kipper and his friends' breakdown.
Kipper and his friends reappear in Captain Underpants and the Revolting Revenge of the Radioactive Robo-Boxers. After witnessing an altered time period and surviving an attack of giant zombie nerds, Tippy Tinkletrousers went back to the past and froze Kipper and his friends with the Freezy-Beam 4000 before they could see the original Tippy. Tippy then shrank the original Tippy and took both of them four years into the future. Kipper and his friends then unfroze and continued running away. As they did not see Tippy as they were running out, the timeline was restored to the original state.