"Do you know, I still have trouble believing it. Of all the people to go over to the Dark Side, Sirius Black was the last I'd have thought... I mean, I remember him when he was a boy at Hogwarts. If you'd told me then what he was going to become, I'd have said you'd had too much mead." - Madam Rosmerta (PA10)
Madam Rosmerta is the proprietor of the Three Broomsticks in Hogsmeade. She is at least in her 30s and likely a bit older, since she was running the Three Broomsticks already back in the 1970s. She is well known by witches and wizards from all over, including the teachers from Hogwarts, rowdy groups of warlocks andgoblins, and even the Minister for Magic himself (PA10).
She has been described as "a curvy sort of woman" (PA10) and "pretty" (PA10, GF19). Ron gets a bit red in the face when he gets near her (PA10), a fact which Hermione is only too happy to point out (HBP12).
Rosmerta's life took a dark turn during the 1996-1997 school year when she was put under the Imperius Curse by Draco and/or the Death Eaters trying to infiltrate Hogwarts. One of the things we know she was forced to do was to put a curse on Katie Bell so she would carry the opal necklace to Dumbledore (instead,Katie herself was injured) (HBP27).
'Rosmerta' = a Celtic goddess of fertility and wealth. JKR chose the name because it means "good purveyor." [JKR: "More idle jottings (page 1)"]
In the movie Harry Potter and the Prisoner of Azkaban, Madam Rosmerta is played by Julie Christie.