How is Maria from the Twelfth Night a stock character?In your answer, be sure to define stock character.

A stock character can be considered a stereotype, and in
drama, a playwright employs stock characters to further the complications of the story. 
Stock characters are used especially often in Comedies, as it is the recognizably
stereotypical human foibles that make for good comic characters and situations --
foibles like craftiness, dimwittedness, miserliness, and  being crazy-in-love or a
braggart.

Maria is an embodiment of the "crafty servant"
stock character.  This character was found in Comedies of ancient Rome, but more
contemporaneously to Shakespeare, the Commedia dell' Arte of Italy.  This comic and
improvisational theatre form allowed both male and female performers onstage and might
have been a very strong influence on Shakespeare's theatrical sensibilities.  The comic
servant from Commedia who can be associated with Maria is Colombina, the crafty servant
of the female lead of a play, a servant who also often had a love
interest.

For more on stock characters, Commedia, and
Maria, please follow the links below.