Onion Knight: 
Let’s start with the useless job. You need a Level 6 Squire and Chemist to unlock it. For those of you who are familiar with the Onion Knight from previous FF instalments, you know what to expect. Sporting its typical bulbous plumed helmet and puffy outfit, this job is useless in its raw form. It can equip anything – to the extent that male Onion Knights can even equip female-exclusive gear –, but has bad stats and no abilities to its name. What’s worse, it doesn’t gain any EXP from regular combat. So how do you level it then? With a lot of jumping through hoops. Each two jobs a character masters gains them one level as an Onion Knight. So to get it to Level 8, which is where it becomes worthwhile, a character would need to master 16 (!) jobs. But that’s not all. Since the Onion Knight can’t gain EXP in combat, the only way to level it up to take advantage of its amazing Level 8 stat gains is…to use the Wild Boar’s Bequeath Bacon ability. Considering the effort required to even obtain a single Wild Boar, this is, frankly, just taking the piss. Oh, and one more thing. As its predecessors in the series, the Onion Knight becomes even more powerful by equipping the exclusive Onion gear…which is only available via multiplayer. Basically, unless you’re a masochist, stay very far away from this job.
Dark Knight: 
A much more interesting addition than its veggie counterpart, the Dark Knight basically looks like a Black Mage in black armour (which is especially cute with the female characters’ braids). While it has the average mobility and speed of a Knight, it also has the highest attack power of any generic job. It can equip heavy armour, robes, swords, knight swords, axes, flails and fell swords, which are only available via multiplayer, but aren’t a big loss. Its Action Ability is called Darkness and includes Crushing Blow, which can inflict Stop; Abyssal Blade, which damages multiple targets depending on their proximity to the user at the cost of 20% of the user’s HP; and Sanguine Sword, which drains HP from a target. The other perk of this job is that it allows female characters to learn Move+3 as a Movement Ability (male characters get Jump+3 instead), something which they couldn’t do in the original game. It also has a Support Ability called Vehemence, which increases the effectiveness of attacks and healing abilities by 50%, but also increases damage received by 50%. Risky, but potentially devastating. Of course, it all comes at a price: the Dark Knight is a royal pain to unlock. Not only do you need Level 8 as a Geomancer, Dragoon, Samurai and Ninja, you also need to master Knight and Black Mage…AND kill 20 enemies with the character in question (i.e. have them turn into crystals/chests). Honestly, by the time you do all this, your characters will probably be powerful enough to plough through most enemies with ease, so is it really worth it? I’m not sure. On the other hand, if you intend to try out multiplayer, raising a couple of Dark Knights will definitely be a good investment.