The Merc is a secondary character with its own set of stat enhancing skills and items.
To access the merc panel click on the helm icon or hotkey (P)
Click on the status bottom to see the merc stats and to see the skills, click on the skill bu
Now as players can see, the merc can be a different class to the main character or the same. Using a different class can be better or worse depending on the dungeons and opponents.
The skill panel features the merc upgrade cards. They range from O to 3.
Be careful that you do not go by the 0-3 as a guide to what is better, as the range of cards is higher and a o card may have a better bonus than a 2
The skills can be upgraded using items at the blacksmith whom in in the center of town, click on synthesis which is 4th down on the list. then merc card which is 4th down on the second list.
It will show the items required to make each card... unused or unneeded cards can be broken down at the blacksmith by clicking on break down which is the 3rd bu