Charlie and Violet are Elena's twin baby brother and sister. They are introduced in "Keeper of the Lighthouse" (in the comic), after Zick and Elena delivered Lardine and her monsters to their new Detention Oasis. Charlie is actually a reincarnation of Elena's imaginary friend, Charlie Schuster, who disappeared after that adventure.
Charlie was kidnapped by Moog Magister in order to power his machine, by harnessing his powers as an imaginary friend. In the episode "Trust Me", they were the reason Zick's Dombox got into Harvey's suitcase.