The Onion Knight is the most iconic job in Final Fantasy III. Its availability varies depending on the version: In the 2D versions, the Onion Knight is the starting job. In the 3D version, this role was given to the Freelancer, although the Onion Knight wasn't removed; It appears as a hidden job unlocked by completing Topapa's request via Mognet. It is the only job in the 3D remake to be optional.
The Onion Knight, on the surface, is a very underwhelming job. Statwise, it starts with 5s across the board and increases all of them by 1 every 5 levels (i.e. at level 1, all of the Onion Knights stats will remain at 5 until level 6 at which point they remain at 6 until level 15, where they increase to 7, etc). This more or less encourages the player to get through the opening minutes of the game to get to the jobs as quickly as possible and makes it unlikely that a player will use it beyond that point (in the Famicom version: In the remake, it's an even harder sell due to coming later in the game).
Of course, that's not the whole story
Around level 90 (FC)/93 (3D) the Onion Knight's stats start ramping up, going from the lowest stats to the highest by the time level 99 is reached, having maxed stats.
Another quirk of the Onion Knight is a set of rare equipment in the endgame known as Onion equipment: As the name implies, only Onion Knights can use these items. The catch is that these can only be gained as rare drops from dragons in the Crystal Tower, some of the fiercest enemies in the game.
The 3D remake makes some interesting changes. Now, Onion Knights can equip almost anything on top of retaining their unique equipment. In addition, they can now wield magic, although their MP growths are less impressive than those of the mages. Lastly, the new bonus dungeon (known in-game as "???") exclusively has Dragons as random encounters, making it an ideal location to farm Onion equipment.
Still, the Onion Knight's abysmal base stats means that it lives and dies by the equipment it has.
The Onion Knight has access to all 8 tiers of magic. However, later tiers have much lower casts (tier 8 only has 4) meaning that an Onion Knight can't rely on their magic, making them similar to Red Mages.
The guide highlights that the Onion Knight's growths will cripple it. Thus, it recommends making a Monk or Black Belt into an Onion Knight after their HP value is maxed out, or switch from Onion Knight to Monk when close to a level up.
The guide heavily stresses that Onion Knights only come into their own during the 97-99 range. A low-level Onion Knight requires a more well-rounded team (healer, two damage dealers or a damage dealer and a strong magic dealer). Prior to the aforementioned range, Onion Knights rely heavily on attack magic, in-turn emphasizing item magic.