"Garrett" is a clone spy and saboteur created by the witch Haggar to infiltrate the Castle of Lions by pretending to be Coran's long lost son.
Using her magic and the machine she'd used to create a clone of Lotor, Haggar created Garrett. He was then deployed to Arus, where he met Allura after her lion malfunctioned and crashed.
Garrett gained the confidence of the Voltron Force, and even trained as a reserve pilot. He was assigned to the control room. He then poisoned Coran to take him out of action.
Garret eventually revealed his true nature and intentions to Coran and the Voltron Force, wanting an army of clones with himself as leader.