Mimi

From Paper Mario Wiki

Mimi is the second Boss in Super Paper Mario. She is the shapeshifter in Count Bleck's army. She often changes into other characters such as Merlee, Merlon, Bowser, Peach, O'Chunks, Count Bleck, and the King Sammer. She can also transform into a giant spider which she calls her true form(most likely not her true form since she is the girl form when she is defeated and during most of the game, she calls it that most likelt due to that being her main fighting form). It is said she has a crush on Count Bleck and her idea of a perfect world is that their were hunky lifegaurds around her.

You fight her three times. You face her as the Boss of chapter 2-1. She starts the battle invincible, but Merlee's cheers break the invincibility. You face her a second time in 6-2, where she is in her normal girl form. She loses but reveals that she was just wasting your time. Peach does her final battle against her in 8-2. She is saved by Peach after it and she helps reform the Pure Hearts.