The Dark Fighter, also known as Fighter in the NES version or Dark Warrior in the GBA version, is an enemy from the original Final Fantasy. Despite their namesake, they are, in fact, more of a support spellcaster, and frequently show up alongside other monsters - typically Dark Wizards, but occasionally other foes. It is recommended to target them first, as they can heal party members, buff them defensively, provide heavy magic resistance to a teammate, or dispel allied buffs - which is not something you want against say, Dark Wizards.
Thankfully, their HP is unimpressive and their defense is mediocre, and they will go down quickly if targetted. The key is not letting them stick around, lest they make a fight far more difficult.