A Mercenary is a class of soldiers that use swords only. They tend toward balanced statistics, although they generally have low resistance. Their defense is usually higher than a Myrmidon, but they may not be as fast. They have very high skill and decent strength, along with better HP than the Myrmidon. Their Luck does suffer sometimes, though.
Mercenaries are some of the most powerful fighters you can have on the field since in most cases they can deal lots of damage while being able to take a few hits as well. Their one problem is that once they are surrounded by enemies, they can't leave the battlefield since their Movement is mediocre. However, a high-level Mercenary can dispatch hordes of axe users with ease, and even stand a chance of avoiding lance and sword attacks. They will more often than not strike their target, and score a few Critical Hits due to their high Skill.
Josh is the only playable Mercenary. He automatically promotes after chapter 16. There are very few NPC Mercenaries (the generic Mercenary Array does not count).