Maria is Olivia's lady in waiting or, her "chambermaid" (but not the kind of chambermaid who scrubs toilets and changes sheets). Though we never see Olivia and Maria giggling about boys and painting each other's toenails, we know that Maria is very close to Olivia – close enough to know her lady's handwriting and close enough to forge it convincingly. That's exactly what Maria does when she writes a letter that brings about Malvolio's downfall.
Why does she do it? As Olivia's lady in waiting, it's her job to keep Toby Belch and company from being too rowdy while Olivia mourns for her dead brother. At the same time, Maria's a girl who likes to have a bit of fun herself. She's saucy, sharp-tongued, witty, and knows how to put the drunken noblemen in their places. She also knows how to tell a dirty joke. So, when she gets fed up with Malvolio's constant criticism, nagging, and judgmental ways, she masterminds a plot to teach him a lesson.
Her scheme to humiliate and punish Malvolio is so clever that Toby Belch falls in love with her and the two get hitched (off-stage) by the play's end. We sort of see this coming, since Toby's always bragging to his drinking buddies that he's got a little somethin' going on the side with Maria. There's some debate about whether or not Maria is a social climber. Some study guides will tell you that Maria's got her sights set on marrying up in the world. We're not sure if that's true, but the idea likely has to do with the fact that Toby says, "I could marry this wench for this device […] And ask no other dowry with her but such / another jest" (2.5.188-189). In other words, Toby suggests he'd marry Maria in a heartbeat and would forgo a dowry if she could come up with another great prank. OK, this may suggest that Maria doesn't have much of a dowry, and we know she's not as high ranking as Toby, but that doesn't really make her a gold-digger now does it? That said, we do think the play is awfully concerned with social ambition, which you can read more about by going to "Society and Class."
Maria's not a big drinker or a rowdy party girl but she is one of the play's unruliest figures. Along with the rest of the zany crew (Toby, Aguecheek, Fabian, and Feste), Maria helps establish the play's festive and rebellious spirit.