Emet, also known as the Iron Giant, is a robot created around the time of the Void Century who was a friend of Joy Boy. Since that time it has been mostly de-powered, only reactivating in two known instances, the first of which occurred 200 years ago when it nearly attacked Mary Geoise. Afterwards it was smuggled away and kept on Egghead, where in the present day it was used to guard Vegapunk's prerecorded video being broadcasted from within itself. After hearing the Drums of Liberation from Monkey D. Luffy, Emet powered back up and aided the Straw Hat Pirates in battling the Five Elders. (Source: One Piece Wiki)