Violet McGuffin was a substitute for Rose Tyler in a hypothetical version of history where Rose had saved the life of her father.
Biography
In a hypothetical version of history where Rose Tyler successfully saved the life of her father in 1987, the changes were substantial enough to the timeline that time created substitutions; Violet McGuffin took Rose's place, moving into Powell Estate with her family after the Tylers moved out. Violet dated Mickey Smith, got a job at Henrik's and began travelling with the Ninth Doctor after defeating the Nestene Consciousness.
In a further hypothetical situation, the timeline would return Earth to its "regular course of history" perhaps centuries after Violet and the Doctor travelled to Victorian Cardiff and defeated the Gelth, and to 2006 to defeat the Slitheen and convince Caroline and Mickey she was alright. (GAME: Father's Day [+]Loading...{"page":"119-120","1":"Father's Day (game)"})
Personality
Violet was brash and opinionated. (GAME: Father's Day [+]Loading...{"page":"119","1":"Father's Day (game)"})
Behind the scenes
Violet McGuffin is evidently named in reference to the literary trope known as the "McGuffin"; like her namesake, Violet serves a role in the suggestions to the players of Adventures in Time and Space: The Roleplaying Game of how handle repercussions of changing history without being truly distinctive unto herself.