The character of Claire Redfield did not exist in the early draft scripts for the film and previously Jill Valentine (who appeared in Resident Evil: Apocalypse played by Sienna Guillory) would continue the role in Extinction. Later producers Paul W.S. Anderson and Jeremy Bolt decided to have a separate game character appear along side Alice saying, "We thought, rather than bring Jill back, put her with another game heroine."
http://en.wikipedia.org/wiki/Resident_Evil:_Extinction