Cat Mercenaries
Cat Mercenaries are NPC's that fight along side you and start out with their first skill unlocked.
This feature is unlocked when you progress the main story far enough to receive the Cat Mercenaries quest. Complete this quest to unlock the first couple of cats and speak to Maysa and Po to unlock those last two.
To access the mercenary interface tap the More button then select Cat Mercenaries.
Accessing the Cat Mercenaries Interface
Hiring Mercenary Cats
From here you can view each cat and hire them for either 1 day for 10,000 zeny or 6 days for 60,000 zeny. Mercenary Coupons are worth 10,000 zeny and will be substituted for every 10,000 zeny so if you want to rent Goro for 6 days and have the tickets then 6 tickets will be consumed instead of zeny.
You can hire up to two cats initially but you can increase this limit to all four cats with an Adventure Skill when you raise your adventure rank high enough.
Example of the Cat Mercenaries Interface, before clicking on Renew to hire him
After clicking Renew, rental options are presented
Adventure Skill that increases this limit to four
The item shop in your Pet Labor system loves to give out discount coupons that allow you to hire these cats for free so eventually you'll have tons to work with.
Unlocking Additional Mercenaries
To unlock the other cats simply talk to them in eastern Prontera City. If you don't remember where you recruited them and they are still locked then open the Cat Mercenaries screen and select them from the dropdown to the left of Goro's portrait (see below). Click on the Go button and it will take you to the cat where you speak to them and unlock them. Make sure you talk to each of the cats while you're there so you unlock all four.
Click the small arrow to the left of the large cat portrait to expand the list of mercenaries
After clicking on the icon to select Wasabi, the tank mercenary, click Go to head to the unlock quest
Unlocking and Upgrading Skills
You may unlock and upgrade skills by purchasing cat costume items from the KittyCat Shop in Prontera on the far east side. Most of the pieces either Eden Coins but some cost Kitty coins. The shop offers a quest every couple days that rewards you with 4 Kitty coins, enough to buy one more costume each time you finish the quest.
Example of Goro's "Tuna Meal" ability which revives you if you die. Great to keep you farming if an MVP comes over and kills you while you're away.
Example of Maysa's "Cat Fragrance Field" ability which provides buffs to all allies standing in the field
Example of Goro's "Saury Meal" ability which provides an SP recovery buff to the party which will be useful while farming if your skills eat through your SP reserves
As you can see above, some of these abilities can be quite useful while farming. Sometimes these cats can even go into instances with you though bringing other players is always preferable, sometimes bringing Wasabi to tank damage for you is incredibly valuable.
Cats have their own level too, represented by "Mercenary Level" which caps at 10. These mercenary levels unlock the passive skills on the bottom section of your cats skill list and raises the base stats of your cats.