Head Rush.
Nobody ever wanted her, just look at these shelves that are full of her!
Her attacks aren't good, they don't synergize with each other at all. She's big, clunky, and you need to take needless risks when playing her since her whole moveset revolves around getting up close to the enemy.
Her design is also ugly.
She probably would've been better in Swap Force to be honest, since that game's combat is based on knocking down the enemy before they can attack you, which is something Head Rush would be great in.