Jemma Spark has used many of Ruby Ferguson's original characters and introduced new characters in all the books in her Jill series. In her tenth book Jill and the Prize Winners Jemma introduces characters from other pony books: Charles Ravenscroft from Showjumping Secret by Josephine Pullein Thompson, Janet Fawley from Janet Must Ride by Diana Pullein Thompson, Lettie Lonsdale from Pony For Sale also by Diana Pullein Thompson, Rennie Jordan from Rennie Goes Riding by Monica Dickson, Patrick Huntingdon from Prince Among Ponies by Josephine Pullein Thompson, and Susan Barrington-Brown from the Barsetshire Pony Club series by Josephine Pullein Thompson.

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The idea of incorporating characters from other pony books is carried on in her book

 The Jill Crewe Miscellany No. 1 there is a compendium of all the characters in Jemma's books to this point. There are also two short stories that include some of the new characters introduced in Jill and the Prize Winners and introduce several new ones including Noel Kettering, Christo Carstairs and Henry Thornton