Fiddle is a cat who was created when Cat Chant's sister Gwendolen got tired of listening to Cat practicing on a violin.  She turned the instrument into the cat who became named Fiddle.

Shortly after Cat and Gwendolen took up residence in Chrestomanci Castle, someone came round the neighborhood and purchased Fiddle for a small sum, after which he became a resident of the castle, mainly wandering about in the gardens.

During the climactic battle that nearly results in the death of Cat, Gwendolen announces that she used one of Cat's nine lives to turn the fiddle into a cat, so without it in hand, the villains determine they can't fully kill Cat as a sacrifice.  Millie rescues Fiddle and goes on to save the day.