Unknown, but some good guesses can be made simply at her job and where she works. She was plausibly affiliated with Ratigan and his gang. However, she was also plausibly fearing them.
Role in the film
Disguised as sailors to find Ratigan's repair, Basil and Dr. Dawson interrogate the waitress who pretends, after a long hesitation, that she doesn't know of him. During Miss Kitty's show, she denounces discreetly the two mice to her employers, who puts drugs in Dawson's beer glass.