Hannah Baxter is the lead semi-fictional character and protagonist of the British ITV2 television series Secret Diary of a Call Girl. Hannah secretly has a double life as a high-class call girl as her alter ego Belle.

The character Hannah Baxter is based on the real life published memoirs of former London call girl Dr. Brooke Magnanti, who is better known by her pseudonym, "Belle de Jour". The character Hannah is portrayed by Billie Piper.

Hannah is a university graduate with a degree in English literature, however chooses a career as a call girl for the sake of the money. She reveals that she makes a yearly turnover of £105,000 (though, she claims, it takes £20,000 "to be Belle"). She further claims her career is ideal since she is "fundamentally lazy", in control, and also because she genuinely enjoys the pleasures of sex. Open minded and truly a professional when it comes to her role as a high end prostitute, Hannah has gone as far as considering a breast augmentation to better appeal to clients, and will likely attempt a variety of fetishes and bedroom scenarios with them. Furthermore, Hannah wants to pride herself on her skills in the bedroom and her ability to size up her male clients, such as predicting what kind of lifestyles they lead based on what their sexual interests are and the price, ratings, and styles of the hotels they pay for on "out calls".

Hannah is typically capable of separating her true self while donning her alter ego while working; however, there have been moments in the series where she's found it difficult, such as when she lets her guard down and reveals to a client in the first episode her real name, has intruding thoughts of family and relationships at times with clients, or when she develops a friendly relationship with a male client only to feel personally betrayed when he chooses to hire a different call girl. Despite the disappointments and emotional frustration that sometimes comes with Hannah's work, she is truly happy with her work, as demonstrated at several points throughout the series when she attempts to get a "normal job".

Hannah is portrayed as a very wise, confident, intelligent, and at times, an outspoken or cheeky character. It is demonstrated throughout the series that Hannah is very meticulous when it comes to organizing her private life with her work, for which she keeps the two completely separate, and will go great measures to keep her occupation concealed from family and friends, which often means she relies on lying, such as telling others that her occupation is a night-time legal secretary for an international law firm. Despite Hannah's discretion and lengthy attempts to keep both worlds apart, the issue of secretly juggling both identities frequently reoccurs in the series, often resulting in negative inner turmoil for her and feelings of loneliness.

Often Hannah deals with how much balance is placed on either her personal life or work, for which she is, usually, focused on work – leaving her personal life sometimes neglected, such as when she misses the birth of her sister's child. She claims that romantic relationships are typically avoided, since her profession merely complicates them, which is demonstrated in the second series where she develops a relationship with a man that ends badly. On the show there are visual differences between her and her alter ego Belle, for which she maintains a girl next door image in her real life, and an elaborate sultry one as Belle, which usually includes provocative clothing and impeccable makeup application.

The first season serves as an introduction to Hannah's double life as Belle, her variety of clients, rules, and methods of juggling her separate worlds while trying to please her clients, and her sometimes unsuccessful attempts to remain connected to her best friend, Ben (Iddo Goldberg), and family, who are not aware of what she does for a living.

Eventually, after a couple of incidents where Hannah becomes troubled with work and suffers the burden of secrecy by having no one to talk to, she reveals to Ben what she does. Although Ben is disappointed and angered that she has been deceiving him, he accepts Hannah's choices, counsels her on matters of work, and even joins a foursome with her and a married couple who are her clients. It becomes apparent that Hannah and Ben – who used to be in a romantic relationship two years ago – still may have more than simply platonic affection for one another. Towards the end of the season Hannah is offered a role as a courtesan, which she is initially enthusiastic about, but eventually walks away from – despite the luxury – because of the lack of excitement. Instead of returning to her agent Stephanie (Cherie Lunghi), Hannah chooses to go independent, with the support from Ben.