Bowser is the villain of Mario & Luigi: Paper Jam. In previous Mario & Luigi games, Bowser is often co-villain, pairing up with someone else, sometimes against his will. He is possessed by the ghost of the witch Cackletta and becomes Bowletta in Superstar Saga. Bowser is possessed by the Elder Princess Shroob after eating a mushroom in Partners in Time. In Paper Jam, Bowser and Paper Bowser form an uneasy alliance, but I found myself wishing Bowser would pair up with someone else, someone who would make Bowser unstoppable.
That someone is Toadette.