Tostada is a female luchadore who aids Juan Aguacate in cooperative play. She is the guardian of the mask that Juan wears. When a second player joins the game they will play as Tostada as a default. However, the first player can also play as Tostada by accessing the costume portion of the store. When Player 1 is playing as her, her clothing turns blue.
In life, she was known as Josefina, as revealed by an elderly man in Canal del Flores recounting the actions of a heroine who helped evacuate Camposanta after the eruption of the Pico de Gallo. For her heroism she was evidently granted a different state of death in the same manner Jaguar Javier was, and set to watch over the mask that grants Juan his power.
She seems to be extremely attuned to the mask, as whenever Juan, it's wearer, learns a new move, she does as well. The origin of the mask and her connection to it is unknown, as her actions at Camposanta did not involve a mask of any sort.
She can be seen mingling with Juan in the credits, presumably as the mask is bound to Juan for life and she is similarly tied to the mask. After Juan's death the mask--and presumably Tostada---disappears, perhaps to wait for another hero to aid in the future.