You need the Elusive Badge for this optional party member
How to get Ms. Mowz in Paper Mario: The Thousand-Year Door (Guide)
Most of Mario's companions in Paper Mario: The Thousand-Year Door will be unlocked as you complete the story, but there's actually one optional companion that you might end up skipping if you're not paying attention: Ms. Mowz, the awesome mouse thief, requires a short side quest to unlock.
Mrs. Mowz is great for literally sniffing out treasure and stealing loot from enemies, so if that's of interest to you, you'll want to add her to your party. Below we describe how to obtain Ms. Mowz in Paper Mario: The Thousand-Year Door and where to find the Elusive Badge she needs.
To unlock Ms. Mowz you must have at least gotten the 4th star from Creepy Steeple. After this, her request will appear in the Trouble Center (the building with the sheet of paper on it in eastern Rogueport). The request is "Elusive badge!" and it is requested by "???."