Kurt Wimmer has stated that the Ultraviolet film was directed to allow for an alternative theory about the story. In this theory, this the character of Six is actually Violet from the future, who comes back after a cure for hemoglophagia has been developed. This explains how Six has the cure and nobody else does. It also explains why Violet is so adamant about protecting a human, despite her goal in life being to kill them all (although she doesn't know Six is her from the future, she can just feel it).
The theory also explains why Six often disobeys Violet and leads where they go. Being Violet from the future, Six remembers how things happened in the original timeline, and can prevent them from occurring this time around.