In episode 20 after returning from visiting Hakin in India, she comes back decked out in their clothing.
Monica Campbell is Eleanor Campbell's older sister. She randomly shows up in episode 16.
She mentions getting married and invites Eleanor to visit her at the "Mildrake mansion" so she likely married into that family and her name may now be Monica Mildrake.
Eleanor says she doesn't want to impose on her brother in law Fredrick likely making her husband Fredrick Mildrake.
When she arrives at the Jones mansion, she hands a card to Stevens which he reads as "Countess Mildrake". She also detests William and considers him unfit to be with Eleanor.
She is voiced by Kikuko Inoue.