Baby Wario is Wario when he was a baby. In Yoshi's Island DS, he (and multiple other famous characters from the Mario franchise) will help Yoshi save all of the babies that were recently victims of a mass-kidnapping started by Kamek. He will join Yoshi's team in world 3, though will shortly leave due to his greedy nature. He eventually returns to help the babies one last time in World 5-8 to defeat Bowser. After Baby Luigi and all the other kids are rescued and returned to their homes, Baby Wario appears to be an orphaned homeless baby. At the end of the game, it is revealed that Baby Wario, along with Baby Mario, Baby Peach, Baby Donkey Kong, Baby Bowser, Baby Luigi and Baby Yoshi are the seven star children.
Oddly, at GDC 2007, Charles Martinet, the voice of Baby Wario, seemed to have forgotten that he voiced him, as he was quoted saying how wishes to some day do a Baby Wario.