Since she is a female, everyone believes that Emilia is unintelligent. Emilia is looked down upon because she is a woman and women are inferior to men. Although on the surface, Emilia seems to fit the "dumb wife" description, her intelligence and ability to slowly uncover Iago's master plan proves this image to be extremely false.
She pieces together small clues to form the basis of Iago's plan to seek revenge on Othello and Cassio. After Emilia steals Desdemona's handkerchief for Iago and asks him the purpose of it, he tells her "be not acknown on't/I have use for it" (III.iii.367-368), yet says no more.
Emilia also appears to be a loyal companion to Desdemona. After retrieving the handkerchief for her husband, Emilia tells Iago, "if it not for some purpose of import,/Give't me again. Poor lady, she'll run mad when she shall lack it" (III.iii.364-366). Although she puts her husband's priorities first, Emilia still proves loyal to Desdemona
As one can see, Emilia's contradictory nature contributes to her complexity. She seems to deceive others because she does not broadcast her thoughts and emotions as others do. Her intelligence is overlooked by even her husband, as he does not assume she will figure out his plan.
Emilia, in some ways, is very loyal to Desdemona and tries to get her to realize that men are cads and that Othello CAN and is a jealous man; at first, Desdemona will have none of it, but she slowly realizes that Othello is jealous after his attitude changes towards her. Emilia is married to Iago and they clearly have a stormy marriage. Iago treats her poorly and verbally abuses her. Emilia, however, is no "shrinking violet." She is "street smart" and is a strong, opinionated woman in her own right. However, she is limited because of her gender, as all women were. Iago treats her as a trivial creature who is a nuisance for him. However, she ends up holding the key to his evil plan to make Othello jealous: the handkerchief.