Mercenaries (or “Mercs” for short) are soldiers for hire. Like pirates they are loyal only to their own code and cash, but instead of working for themselves mercs rent their services out to the highest bidder as assassins, thieves, fighters, hired muscle/protection, bounty hunters, or drivers/transportation. Mercs may have their own moral codes, but many are willing to bend theirs for the right price.
Mercs come from a variety of backgrounds and races, and may specialize in a particular type of job that fits them best. Fast reflexes make an excellent getaway driver. Nimble fingers make an amazing thief. Strength gives a merc an edge in a fight. An uncharismatic merc can make an intimidating bodyguard.
Because mercs are willing to do things other characters might not, they can find it hard to make friends. Other classes are, rightfully, a little afraid of mercs when they meet them. When cornered in a fight, a merc might have only themselves to rely on.
Choose one ability and add a +2 to it