Dida is Vitangelo Moscarda's wife, the person who sets off the novel's crisis with a remark about her husband's nose and who, for most of the book, is the closest witness to everything he does. The book does not make her a villain: she truly loves the husband she herself built.
The daughter of a man who appears in Book VI, Dida married Moscarda when he was twenty-three. She calls him Gengè every minute, says she knows what he likes and what he does not, and in Chapter XII of Book II shows how: she undoes the girlish hairstyle that pleased them both, because it must please not herself but her Gengè. She is sincere with him, wholeheartedly, and for that reason Moscarda cannot bring himself to call her false.
In Book III, she laughs out loud when he asks her what his profession is, and the laughter undoes the horror without giving him anything back. In Book V, after Marco di Dio's eviction, she treats the act as a prank by her Gengè and scolds him. In Chapter VIII, when he complains of being called a usurer, it is her laughter that wounds him at his vital point, and only then does he order the bank closed. He pushes her into the armchair, and in Book VI she leaves with Quantorzo, leaving orders for her things to be taken to her father's house.
Only later, through Anna Rosa, does Moscarda learn that Dida considers him a fool and that she, her father, Firbo and Quantorzo gathered evidence to have him committed. Even so, he concludes that with her Gengè she was not pretending. Everything known about Dida passes through the gaze of Moscarda or of Anna Rosa.