Muggles' Guide to Harry Potter/Characters/Madam Rosmerta

Overview
Madam Rosmerta is "a curvy sort of woman with a pretty face", the owner of The Three Broomsticks pub in Hogsmeade.

Prisoner of Azkaban
Madam Rosmerta is first mentioned by name when Harry escapes from Hogwarts to join Ron and Hermione in Hogsmeade. She serves Professor McGonagall, Professor Flitwick, Hagrid and Cornelius Fudge, and is invited to join them for a drink. She is unhappy that there are Dementors in Hogsmeade, as they have searched the pub twice and scared her customers away.

Rosmerta finds it difficult to believe that Sirius Black went over to the Dark side, as she knew Black when he was a boy. When Professor McGonagall asks her if she remembers Black's best friend, Madam Rosmerta remarks how you never saw one without the other, eventually revealing that Black's best friend was James Potter, Harry's father.

Goblet of Fire
Madam Rosmerta isn't impressed that Professor Moody insists on drinking from his hipflask whilst he visits the Three Broomsticks, instead of buying a drink from the pub, and glares at Moody as she collects glasses from the tables around him and Hagrid.

Order of the Phoenix
Perhaps curiously, we do not see Madam Rosmerta in the Three Broomsticks when Harry is there being interviewed by Rita Skeeter. As Harry is then banned from all further Hogsmeade weekends by Umbridge, we do not see Madam Rosmerta in this book.

Half-Blood Prince
Rosmerta greets Harry and Dumbledore when they secretly left Hogwarts to find Voldemort's Horcrux. On their return, she warns them of the Dark Mark above Hogwarts, and loans them brooms to speed their return to the school. At the school, Harry is present, though immobilized and concealed by his Invisibility Cloak, as Dumbledore engages Draco Malfoy in conversation to attempt to win his allegiance. In the course of conversation, Dumbledore wonders how Draco knew when they left the castle, and then realizes that Rosmerta, under the Imperius curse, had told him. This revelation also explains how Katie Bell came to be carrying the cursed necklace back to school after a Hogsmeade visit; Rosmerta, acting under Malfoy's curse, had met Katie in the toilets, placed her under the Imperius curse, and given her the necklace to deliver to Dumbledore.

Relationships with Other Characters
Ron seems to have a bit of a crush on Madam Rosmerta; perhaps he just likes her shape. He is well aware that she is literally old enough to be his mother; in fact, Madam Rosmerta was tending bar at the Three Broomsticks when Harry's father was at Hogwarts.

Analysis
In Harry Potter and the Prisoner of Azkaban, Madam Rosmerta's primary function is to be an excuse to tell the story of Sirius Black's supposed back story. The more mature reader might find this a little unlikely, as a barmaid or publican is likely to know all the news almost before the people it's happened to. This would not be obvious to the book's intended audience, however. Hagrid's unfamiliarity with the story might be reason enough to tell it, but Rosmerta has a vested interest - the Dementors stationed around the village are hurting her business, and the person ultimately responsible for their presence has just walked into her establishment; who better to explain why she has to take a hit in her profits?

One of the things that goes past without significant mention is that Rosmerta is quite literally old enough to be Ron's mother, and he would realize that if he ever thought about it. Rosmerta mentions that she had known Harry's father and the other Marauders, in Harry Potter and the Prisoner of Azkaban. It is unlikely she would remember them so well if she was significantly younger than them, so we can assume that she is roughly of an age, at least, as Harry's parents, or older if, as is suggested in the discussion with Fudge in the Three Broomsticks just before Christmas, she served them in the pub. Ron still seems to be angling for a glimpse of her in subsequent books, despite the conversation having happened in his hearing, and while he was paying attention to it.