Maria is Olivia’s waiting-gentlewoman. Though she begins by attempting to keep Sir Toby under control, she quickly becomes one of his cronies and the brains behind the plot against Malvolio.
Her motives may be mercenary: by the end of the play she will have married Sir Toby, thus escaping becoming a member of the gentry. This is a particularly neat revenge on Malvolio, who pulls rank on her: she achieves exactly what he cannot and becomes his social superior. Whether this is social climber or genuine affection is unknown. Close to her mistress, she is able to imitate her handwriting well enough to deceive the steward, though not Olivia herself. She is clearly a persuasive lady, able to convince Feste to join in on the joke and able to mislead Olivia into believing Malvolio mad.