Take the trouble center quest that is given by a person marked as “???”. Talk to Ms Mowz above the item shop. She'll ask for an elusive badge in Hooktails castle. Go to Hooktails castle to where you fought Hooktail and walk around until a ! appears over you and blow with Madame Flurrie to get the badge. Once you get the badge, go back on top of the cook’s shop and the mouse thief will join your team.