Which brings us to Helena. She's a complicated gal, you see. She's actually Sarah's twin sister clone—the two were born to the same surrogate mother in the UK. But when the surrogate realized that they were experiments, she hastily attempted to split them up in order to keep them safe. Sarah ended up in the foster care system, and Helena ended up at a Ukrainian convent where she was raised by super-crazy radical extremists, the "Proletheans." They drove the idea into her head that she was the "original" clone, and that her duty was to kill all of the "abominations" who shared her face: a.k.a, the other clones.
So, yes, she's completely insane—and majorly dangerous.
Helena succeeds in killing off a few clones, whom we never meet, and ends up shooting Katja, whom we meet for a hot second, right in the head. Sarah, however, she cannot kill, and she realizes it's because the two are twins.
This doesn't stop her from kidnapping Sarah's daughter Kira, though. It does, however, prevent her from kidnapping Kira for longer than five minutes, because that little girl is so damn adorable and Helena probably just wants to be loved. Kira is her niece, after all.
Unfortunately, Kira gets hit by a car like two seconds after Helena lets her go—but she survives! Helena is truly devastated when she sees Kira get hit, and it seems that she's developed a strange obsession with Sarah, whom she calls "seestra." I guess that's Russian for "sister."
It's worth noting that the Proletheans have kept Helena isolated for all of her life, which caused some major damage to her psyche.