Miss Rosalie is a character in The Lives of Christopher Chant. She first appears when Christopher, surly and offended by his sudden magical transport to a future he absolutely doesn't want in Chrestomanci Castle, is led off to meet the current Chrestomanci, Gabriel DeWitt. The lady who does the leading is wearing long swishing skirts, and introduces herself as Gabriel's Chief Assistant.
We learn that she has done her best to set up a room that will be welcoming to a young boy -- someone they've never yet experienced at the Castle, apparently -- and that she is fairly adept at cricket. It's only later that Christopher learns that he has heard about her several times before meeting her.
'Someone Who Was Once the Light of My Life...'Edit
This is how Christopher's spirit traveling partner in crime Tacroy describes the young lady with a harp who used to play him into a trance each time he'd go traveling. Something unspecified happened to part the two of them, and now Tacroy has to make do with various unsatisfactory substitutes who play the wrong music on the wrong instruments and generally leave Tacroy less substantial than he would like to be while in the spirit. Christopher has to keep firming him up through intuitively magical methods, but Tacroy says it's just not the same without his young lady with the harp.