I find that the difficulty does indeed curve awkwardly, but not because of Ghost.
I thought I was choosing a character at first so I ended up going against Ghost the very first time I played and I beat her just fine.
I was surprised to see that the 2nd Boss was able to reflect Red/Gold Shurikens back at you.
I managed to win against all of them in a single go, but the 2nd Boss can outplay the Red Shurikens where as the 3rd Boss only requires you to alternate between Red and Normal Shurikens so that she takes a hit from the slow traveling one as she is reappearing.