Kerstin Gier (born 1966) is a German author of novels for adults and young adults. Her popular young adult novel Rubinrot (Ruby Red) and its two sequels - a series about time travel - as well as her Silber trilogy were translated into English by Anthea Bell.
Kerstin Gier has also published books under the pseudonyms Jule Brand or Sophie Bérard.
Source: Kerstin Gier on Wikipedia