Ms. Hudson works as a maid and sometimes assistant to Sherlock Holmes. Holmes indicates that she has an Oxford don's knowledge of ancient Greek, but she's a complete autodidact. She consulted with Holmes on several cases when he worked for Scotland Yard. Holmes also calls her a "muse" as she consistently finds herself in the company of powerful men in crisis. Watson notices her Adam's apple indicating that Ms. Hudson is most likely a transgender woman.
Watson first meets Ms. Hudson at The Brownstone as Holmes as offered her a place to stay while she recovers from a break-up with David Renkin, a powerful executive. Ms. Hudson was Renkin's kept woman. With the power out and a snow storm pending, Hudson helps Watson make a fire when they are interrupted by Renkin pounding on the door. Hudson tells Watson to let him in but this results in them arguing. After returning from a case in the snow storm, Watson is pleasantly surprised to find the Brownstone cleaned. Holmes approves of Ms. Hudson's method of organizing his many books. The next day, Ms. Hudson leaves for her cousin's and Watson learns that he's hired her to clean the Brownstone weekly. ("Snow Angels")
Holmes tells Watson that he bought a cat named "Purrdy" for his girlfriend Fiona Helbron but the cat didn't like her. Ms. Hudson offered to adopt Purrdy but things did not work out between them so Sherlock had to put an adoption notice on-line. ("Ain't Nothing Like the Real Thing")
Ms. Hudson provides the hiding place for Mycroft after Sherlock discovers Mycroft is being framed for murder and treason. The hiding place has been restored and stocked by one of Ms. Hudson's patrons and she manages the place. ("The Grand Experiment")
She provides the reference for Sherlock to the curator of 39th Street Library, Raphael. Kitty is dispatched to deal with a break-in and the theft of missing maps and discovers a murdered security guard. ("Terra Pericolosa")