Raised in England by her adoptive parents, who were also the scientists who created the clones, Rachel Duncan was raised with the knowledge that she's a clone. Working closely with an organization that wants to advance human evolution with cloning, Dyad, Rachel wants other self-aware clones to support Dyad's mission.
That places her in direct opposition to Sarah and the other clones. She's also quite jealous of Sarah's ability to have children and covets her daughter, Kira, sometimes to a dangerous level.