Master Miis are the hardest level CPU Miis in Wii Party and Wii Party U and the second hardest in New Wii Party and New Wii Party U. These Miis are unlocked after winning a TV Party game against the Expert Miis.
If you are playing by yourself, they will always be your partner in Balance Boat in Wii Party.
The other CPU Skills are Beginner, Standard, Advanced, Expert, and Advanced Master.
List
Wii Party
Akira, Alice, Alisha, Angus, Asami, CallMeCarson, cscoop, Eddy, Emma, Eric, Frederick (2), George, Hiromasa, Hugbox, Jackie, Jarek, Jibreel, jschlatt, Katsuro, Larry the Cucumber from Veggietales, Legosi, Luccas, Lucía, Malte, Marisa, Masa, Matt (Wii Sports), Matt (Dadpleaseno), Mohamed, MrBeast, Pablo, Penny (Stardew Valley), Pierre, Sakura, Slimecicle, Steph, Stéphanie, Sufian, Takumi, Ted Nivison, Thomas the Tank Engine, traves, TWD98, Tyrone, Víctor, Yoko, Zelda
Wii Party U
Araceli, Azami, Barry, Bo-Jia, Clara, Daisuke, Elena (known as Barbara in Wii Sports Club), Hussein, Faustine, Jeff, Jianjun, Joana, John, Joost, Manami, Maximilian, Na-rae, Olga, Pierce, Polly, Rie, Sara, Susie, Vincent, William, Yoneko, Yuehua, Yuya
Newer Wii Party

Top Row: Tatsuaki, Luca, Siobhán, Pierre, Shouta, Shohei, Asami, Pablo, Ren, Michael.
Bottom Row: Misaki, Jackie, Keiko, Fritz, Maria, Alisha, Kathrin, Martin, Rainer, Ryan.