A companion of the seventh Doctor in Doctor Who.
She first appeared in Dragonfire, where she was a teenaged girl from the 1980s who had been caught in a time storm and ended up on Iceworld, where she meets the Doctor and joins him on his travels. It is later revealed (in The Curse of Fenric) that the time storm was created by Fenric so that she would meet the Doctor, as part of a trap created by Fenric.
Ace was just her nickname, her real name being Dorothy. She was an enthusiastic teenager who liked blowing things up, and was rarely without a canister of nitro nine, an explosive of her own invention. Courageous in situations of physical danger, she was also emotionally quite vulnerable, often forced to deal with feelings of betryal when cought up in the Doctor's elaborate schemes.
In the book Love and War, Ace left the Doctor to fight in the Dalek wars when he used her new boyfriend to defeat the menace of the Hoothi, leading to his death. She rejoined him in Deceit after three years of her time, and was now a much tougher and violet character. She eventually left the Doctor's company for good in Set Piece, taking a time travel device of her own and becomming something of a time vigilante, and assuming the name Dorothee.