He is the most common character in the series along with Mario and Jeffy. He is the main protagonist of the Bowser Junior's Summer School videos and The School videos.
In earlier videos, he was the unbeloved son of Bowser, and was usually parented by Chef Pee Pee. In later videos, he is now the beloved son of Bowser and he doesn’t bother Chef Pee Pee now.
His best friends are Cody, Joseph and Jeffy and they hang out with each other pretty much all the time nearly every video.
Bowser JunIor originally also started out as the main antagonist in his debut Baby Mutant Turtles and he would also be the secondary antagonist of Season 5.
Affects on SML (conspiracies)
Many believe that Jeffy had caused the demonetization on the original SML channel, and had them abandon that channel and move on to Super Luigi Logan due to his mature cursing behavior. Logan Thirtyacre confirmed that Cody, Junior, and Joseph had caused the demonetization because they all had that same attitude. Jeffy was a custom made puppet and the the Nintendo Trio were family friendly content and SML was using them in a way that they were very mature. The SML, SBL, SLL and other Super Logan channels have sticked to the Family Friendly Content. But, they have made puppets for all of the Nintendo characters.