Onion Knights are warriors with distinctive, onion-like helmets. In Final Fantasy III, they have an initially low stat growth rate which steeply increases as they approach the maximum level of 99. In the original version of Final Fantasy III, they are the starting class, can use all equipment, and cannot cast spells or use other special abilities. In the Nintendo DS remake, they are an optional class obtainable through a sidequest and have gained the ability to use black and white magic of all levels.
Source from Wikipeida