Ashamed...of myself? I like getting what I want, so I do whatever I need to do to get it. You're the one that's always having to get rescued by boys. THAT'S embarassing! Golly, you sure are a lot of talk when you have your little boyfriends to protect you!
~Mimi to Peach in Castle Bleck, Chapter 8-2
Mimi was formerly a minion of Count Bleck who has the ability to shape-shift into the form of others.In a cutscene, Mimi does not show her true form, and poses as Count Bleck and O' Chunks.
Mimi posed as Merlee's handmaid in Chapter 2. When Mario frees the Gnaw, it will immediately run after Mimi, chasing her and barking, forshadowing about her. Later, after Mario breaks the vase, Mimi will demand that Mario pay 1,000,000 Rubees to pay for the vase. After Mario earns the 1,000,000 Rubees, Mimi will be at the bank room, and take the Rubees from him. She will then go into shock, rise up into the air, and explode.
In the next part, Mimi takes the form of Merlee and tries to make Mario make a deal that he pay 10,000,000 Rubees for the Pure Heart. When Mario refuses all of her offers, she then becomes angry and transforms into her "true form" which is a spider-like creature with part of Mimi's face and one of her pigtails on it. Mario must then flee into the room behind her, and find his way to the bathroom with staying in one room for too long or else she will catch up. After Mario enters the bathroom and finds Merlee, Mimi will appear and take the form of Merlee again. Mimi and Merlee with argue and insult each other for a while, claiming that they were the real Merlee and that the other was a fake. They both eventually decide to let Mario pick the real Merlee, and start a game show. If Mario picks the real Merlee, Mimi will throw a tantrum and say that she had mimicked Merlee perfectly. She will then transform again. If Mario picks Mimi, then she will laugh and say that she had fooled Mario. She will transform either way. There will then be a boss fight with Mimi, and she will fire and drop Rubees at Mario. After Merlee performs her cheer and Mimi is now vulnerable, Mario must jump on her twice to take off one of her legs. Mario must take off all of her legs in order to defeat her. Mimi can also climb onto the ceiling and drop Rubees from above, as well as being able to send a wave of Rubees shooting up from the floor. All of her attacks can be blocked by Peach's parousel.
Mimi's second appearance is in Chapter 6, when she takes the form of the Sammer King. She hides a bomb in the treasure chest she claims to have the Pure Heart in it, and is shocked when the bomb deals no damage to Mario. She then shows her true form, then starts a boss battle. This time she does not transform, and instead rises up into the air with Rubees swirling around her. She can flip into 3-D, and hurl Rubees at Mario that follow him until they hit the ground. Her other attacks include spinning the Rubees faster and getting close to Mario so the Rubees will hit him, and make Rubees appear in the air and shower them over the floor. Mario can use Thoreau to take her Rubees and toss them at her, either hitting her or knocking away some of her Rubees. Peach's parousel can also guard against her attacks.
After the fight, Mimi reveals that she was just stalling them so that the Void would destroy the Sammer Kingdom. She taunts them, then leaves.
Mimi makes her third appearance in Chapter 8 and takes the form of Merlon to get Mario to press a switch which will take him down to a dungeon. Later, she takes the form of Merlee to ask Mario questions about what he fears most. She will then put whatever Mario said he feared in the next three rooms, and in the fourth room, Mimi will appear angry. Mimi will then taunt Peach, saying that she was a lot of talk with her "boyfriends" around. Peach becomes angry and wants to fight Mimi alone. During the battle, Mimi will transform, climb onto the ceiling and drop Rubees, and shoot Rubees from above. She can also have her all but two of her legs temporarily detach, four Rubees surround her head, and she will spin across the room. This attack will go in a vertical pattern, although the line which she is spinning on will also be moving. She can also rain down Rubees above the ground. Peach must defeat her by hitting her with a Rubee to stun her, then jumping on her to make her lose a leg. Peach must take off all of her legs to defeat her, like the first battle with Mimi. After the fight, Mario and Luigi will leave the room. Peach tells Mimi that she has to go somewhere else to be safe, and Mimi asks why she should care. The room rumbles, and the door is shut on the other side. Inside, the floor has partially fallen away with Peach hanging on to one side and holding Mimi with her other hand. The room rumbles again, and Mimi and Peach fall down.
Mimi makes her fourth appearance along with O' Chunks in Dimension D, the dimension that Dimentio created, and which he sent Count Bleck, Tippi, and Nastasia to. She is wearing the same outfit as before, and seems to be very positive, although gets angry and says that Dimentio could not get away with his betrayal.
After the battle with Super Dimentio, Mimi appears again, along with Count Bleck and O' Chunks. Count Bleck creates the door, and they all go in to the wedding area where Count Bleck first tried to get Peach and Bowser to marry. Mimi and O'Chunks are seen with baffled expressions, but then regain their regualr expressions. When Nastasia starts crying, Mimi cheers her up by saying that they could still build the count's perfect world.
Mimi makes her fifth and final, but optional appearance after the game. She will be in Merlee's Mansion yet again, and say the first few lines that she said in Chapter 2. She will also say that Merlee had hired her for real this time, and she was pretending that the mansion was hers while Merlee was away.
Mimi is shown to be happy, positive, and sassy most of the time, but is quick to get angry and transform into a large spider-like creature.
Mimi is also shown to have many different outfits. She can have a yellow-and-white polka-dotted dress with a red bow, a black dress with a white apron, a small blue dress with a white v-neck and a red neckbow, an orange dress with red frills, a red v-shaped belt and a red bow on her head, a white dress with a diagonal golden ribbon on it, a red belt, a gold ribbon collar, and red flowers on her pigtails, a blue striped dress with a magenta tie and a blue-and-magenta hat with a ribbon and bow on it, a two-layered pink-and-cream polka-dotted dress with a red-and-yellow flower on the layer divider, a butterscotch-yellow bead necklace, and bead hairties, a green dress with a red belt and a yellow clasp, a white collar, and a red-and-brown hat, a purple-and-white polka-dotted winter dress, a fluffy white collar, a purple purse, and one purple bow in one of her pigtails, and a red-and-white polka-dotted dress with a red belt and red-rimmed sunglasses. She also is green and often has pigtails.