Kevin and Nigahiga's collabs, in-universe and in Real Life. Too many examples to list.
Kevin's frequent collabs with Justin Chon lead fans to this. It was especially fueled by the stinger of Wingman.
Ensemble Dark Horse: PapaJumba is certainly very popular among the fans, who are constantly asking if he's going to be in another video while Kevin is away from home. To rub it in Kevin's face, PapaJumba had an iPod app before Kevin himself, with all his soundclips and quotes.