Just for posterity, it is entirely possible to style a character with an image (in this case, the Knight Errant archetype) without having to attach them to the corresponding job. XIV is very bad at this compared to prior Final Fantasy games.
Dark Knight refers to a specific job, but not all armored greatsword wielders are Dark Knights. Their powerbase comes from tapping into their emotional drives to augment their attacks, but there's also no hard rules saying the only way to learn this is to join the official Dark Knight faction (which also seemingly doesn't exist in the game itself, as the only allusion to an Order proper is in the fluff on the website!). Their powers get a small adjustment compared to older games, too: It wouldn't be practical for a tanking job to be spending their own HP, so XIV's version of it lacks the self-sacrificing aspect. For now, at least.
Similarly, someone can be a gallant helper with a sword and shield without actually being a Paladin, which in XIV refers to a faction based in or around Ul'dah's Sultanate. White Mages aren't necessarily just healers, they're also custodians and stewards of the Black Shroud. Dragoon refers to both the official Ishgardian faction of spear-trained-dragon-slaying-jumping-people but anyone who has displayed proof to the Holy See can also be ordained a "dragoon" (emphasis on the lowercase).
In short, don't feel beholden to a class just because the game makes you change to it for the sake of your look.
Dark Knight refers to a specific job, but not all armored greatsword wielders are Dark Knights. Their powerbase comes from tapping into their emotional drives to augment their attacks, but there's also no hard rules saying the only way to learn this is to join the official Dark Knight faction (which also seemingly doesn't exist in the game itself, as the only allusion to an Order proper is in the fluff on the website!). Their powers get a small adjustment compared to older games, too: It wouldn't be practical for a tanking job to be spending their own HP, so XIV's version of it lacks the self-sacrificing aspect. For now, at least.
Similarly, someone can be a gallant helper with a sword and shield without actually being a Paladin, which in XIV refers to a faction based in or around Ul'dah's Sultanate. White Mages aren't necessarily just healers, they're also custodians and stewards of the Black Shroud. Dragoon refers to both the official Ishgardian faction of spear-trained-dragon-slaying-jumping-people but anyone who has displayed proof to the Holy See can also be ordained a "dragoon" (emphasis on the lowercase).
In short, don't feel beholden to a class just because the game makes you change to it for the sake of your look.