Jimmy T. said:
I think that the 12th character for Mario Party 9 is Toadette:
"Toadette will be last playable character for Mario Party 9 because, everyone has a pair (as well as Yoshi and Birdo, Koopa Troopa and Shy Guy, etc.) except for Toad. Toadette will the perfect fit for the game as they were friends in Mario Kart Double Dash and Mario Party 7. There are no words to describe this. She has not been seen since Mario Super Sluggers back on August 2008 and this could be her chance to return."
And according to this website's poll, most people (33%) voted for her so far.