Ms. Mowz
In story mode, you will get six characters (Goombella, Koops, Madame Flurrie, Yoshi, Vivian, and Admiral Bobbery). However after Chapter 4, go to the Trouble Center and take the chore of the mystery person. You will figure out that it is the mouse thief and when the mission is over you get the partner Ms. Mowz.