Mercenaries are hirable help that are assigned to the guest spot during battles. They can participate in many difficult boss battles and hasten grinding with powerful attacks.
You can find the mercenary shop by visiting the Killing Touch in the Onyx City 2 area.
Mercenaries have the following limitations:
A mercenary won't work for you until you've proven you can slaughter a large number of specific foes and earned their respect. Visit them in the Mercenary Shop to see which creatures you must kill, and how many, to accomplish this.
Your account will not be credited with a mercenary's respect until you view it in the shop.