She can be unlocked by entering up, down, up, down, X, Z, X while over her character in the character selection menu.
Fracas is the most unruly member of the Shark Attack team, and unlike her other shark comrades, she will "most likely eat your face off". 
Fracas means "a noisy disturbance or quarrel."
Fracas, unlike Mischief and Mayhem (in which you must destroy enemies before the boss), is a "Boss Rush", which means she will only play the bosses. She's also known to be the hardest difficulty of them all.