Nikki Hallander is an antagonist of the Law & Order: Special Victims Unit 2009 episode Stranger.
Before Stranger
Nikki was the middle of three daughters born to Mr. and Mrs. Hallander of Manhattan. In 2005, her younger sister Heather, who was 14, caught Nikki doing drugs and threatened to tell their parents. From what the Hallander family understood, Nikki insulted Heather enough to make her run away. But in truth, Nikki murdered Heather by making her fall to her death through an abandoned chimney shaft. Heather is thought to be still missing 4 years later.
Stranger
In 2009, Nikki was living with her parents, her older sister Erica, and Erica's young son. On this night in particular, a young woman claiming to be Heather knocked on their door claiming that it was her. 'Heather' looked the same (except she was obese during her disappearance and had lost weight) and claimed that during the time that she was missing, she had been enslaved in a soundproof room, almost like a sex dungeon, and was raped every day.
Detectives Stabler and Benson eventually learned that this woman was not Heather, after finding that she has different DNA and her clover tattoo on her left hand was in a different position. Benson asked where 'Heather' got her tattoo; she said from a tattoo parlor, and the real Heather got it somewhere else. 'Heather' was brought to trial for impersonation, before the Detectives learned that the imposter was really imprisoned, and is in fact named Kristen Vucelik, who vanished from Ohio 6 years ago. She was kidnapped, put in a room, and raped by her own father Carl Vucelik, who was under the alias Carl Vasko while living in New York. Vasko/Vucelik was arrested, and Kristen was reunited with her mother.
It seemed that everything was over, but Kristen went to see the Hallander's one last time to apologize. When Stabler and Benson went to the Hallander's apartment, they noticed that Mrs. Hallander is not telling them something. She admits that Nikki told Kristen that she knew she was an imposter, and that if she wasn't careful, Nikki would do to her what she did to the real Heather. Erica is shocked that her mother knew this, but she didn't want to lose another daughter. The Detectives threatened arrest if Mrs. Hallander didn't reveal where Nikki and Kristen went. They are led to the same rooftop where the shaft was where Heather was pushed. The Detectives got Kristen down, but Nikki threatened to jump. Stabler and Erica got her to come down, and she was arrested.