Sunday and her gang lived in an abandoned frat house at the University of Sunnydale. She and her gang would pick off new students each year, to steal their personal belongings. When Buffy joined the University, she took on Sunday, and was beaten terribly, barely escaping. Later on, however, she managed to get back on her game and killed Sunday and most of her gang.
I'm really unsure of Sunday's age, but I doubt she was very old. I found her interesting because she liked to steal people's things rather then just eat them. She was very strong for her age, and was able to really hurt Buffy. This was because Buffy was definitely off her game. She defeated Sunday quite easily the second time.