One thing we learn about Maria in Act 1 is that she is a very devoted guard of Olivia. We learn of Maria's devotedness to guarding Olivia in the very first scene when Duke Orsino's servant Valentine announces that Olivia's "handmaiden," meaning Maria, refused him admittance, saying that Olivia has vowed not to be seen by anyone for seven years while she mourns her brother's death (I.i.26-27). A second place we learn of Maria's devotedness to protecting or guarding Olivia is when see her scold Feste for being gone so long, warning, "[M]y lady will hang thee for thy absence" (I.v.3). Since Maria knows Olivia has been upset by Feste's absence, we know Maria cares about what upsets her, showing us that she would like to alleviate any of her upsets. We even further see Maria's protectiveness when she scolds Sir Toby for his drunkenness and late night partying, warning that Olivia finds his behavior very offensive.
Another thing we learn about Maria in the first act is that she has a strong sense of moral judgement. We especially see her sense of moral judgement when she scolds Sir Toby for his behavior and refers to Sir Andrew as foolish. She particularly warns Sir Toby that his bawdy, excessive drinking "will undo you," meaning cause his death (I.iii.13). She also very accurately describes Sir Andrew as a foolish, wasteful, quarrelsome, coward. Since Maria is so easily able to see these vices in other, it shows us that she has a strong sense of moral judgement.