OK, I'm nearly done with my quest to win a tournament with every character in Simple mode (just need to do Peach, Bowser, Waluigi, and Bowser Jr.). This is actually much harder than Standard mode because you can't use trick shots as a crutch to mitigate the flaws of some characters.
Anyways, I thought Wario was the worst character in Simple mode, but I was wrong. It is actually...
*drumbeat*
Rosalina. She basically has all the flaws of Wario (below average range, bad control, slow speed, a terrible lunge) without any of his power. Her slices are so weak in comparison to Boo's that it is not nearly enough to make up for it. Like, it's literally impossible to get the ball past characters like Chain Chomp and Bowser Jr. no matter what you do. She can't even take advantage of putting her opponents in bad positions because she has no power or control to knock the ball past them before they can recover.
Boo is better than her in every possible way besides range. Similar power and speed, better control (especially with drop shots), far better slices, and hands down the best lunge in Simple mode (to the point where I think it's actually glitched; you can literally chain lunges to each other to go across the entire court and catch balls that no other character could possibly reach).
At least with Wario if I was going to lose it would be quick. Since Rosalina is so slow on everything her rallies can take minutes at a time. I'm so glad to never have to pick her up again.