When Mr. Winterbourne is in Geneva and in Rome, he stays with his aunt, Mrs. Costello, the biggest proponent of the high-class snobbery that tries to keep a girl like Daisy down. Mrs. Costello has a lot of money and a few houses in Italy, which makes her an important node in the social network of Americans abroad.
To Winterbourne, "She admitted that she was very exclusive; but, if he were acquainted with New York, he would see that one had to be" (1.99). So, she knows her social power comes from being a jerk and a snob and deems this power to be more important than not being a jerk and a snob.
A widow for many years, Mrs. Costello's two sons largely ignore her, and Winterbourne fills in for them by visiting her as much as he can. She looks kind of like a poodle in a Tim Burton movie: "She had a long, pale face, a high nose, and a great deal of very striking white hair, which she wore in large puffs and rouleaux over the top of her head" (1.99).
She's often getting sick with headaches and having to lie down and smell camphor, which sounds pretty gross to us. Costello is quick to voice her judgments about every aspect of the Miller family's behavior, and getting her to change her mind about Daisy would be like trying to convince the Donald to occupy Wall Street.
When she's scandalized by Daisy's latest exploits, Mrs. C smells salts to prevent her from fainting. Basically, she spends a lot of time raising her nose at the people she deems beneath her and then using that nose to smell all sorts of nasty stuff instead.