What did Tiffany do to Nica?
One of the Chuckys tells Tiffany to kill her but Tiffany having developed feelings for Nica decapitates the doll and knocks Nica out with a sedative.
After Glen kills Chucky, he breaks down and Tiffany comforts him showing even though she wanted a girl, she still loved her son. Five years pass, and she is happily living in Hollywood, posing as Jennifer, and is raising Glen and Glenda on her own.
Having developed a twisted love for Cult of Chucky's Nica Pierce, Tiffany sought to keep her around while also diminishing the threat posed by Chucky taking control of her body again. To that end, Tiffany had Nica's arms and legs surgically removed.
Following the horrific massacre in Curse of Chucky, Cult finds Nica committed to a medium-security mental hospital, where she is thoroughly convinced that the idea of a living doll with an insatiable bloodlust was nothing more than a manifestation of schizophrenic tendencies.
Charles and Tiffany jump onto the bed and start making out. He tells her his name is Charles, to which she remarks he should go by "Chucky". In return, he mentions she should dye her red hair blonde. After their first encounter, the two decide to become a couple.
The actress playing Nica, Fiona Dourif is Brad Dourif's daughter (Chucky). Nica is also, unfortunately, one of Chucky's intended victims who is punished for his crimes. As neither Kyle, Andy, nor Ronald were ever prosecuted or imprisoned for his crimes.
Foley (and the "sane" patients) doesn't believe Chucky killed Nica's family, but Nica herself, and uses the killer doll story to hide the shame. It is later revealed that the doctor had been sexually abusing Nica during their sessions through hypnosis, leaving Nica with no memory of it.
When Nica asks Chucky what his motive is, he reveals that he was a family friend that was secretly in love with Nica's mother Sarah. He killed her father and kidnapped Sarah.
He flirts with her and gives her a package for her mother, Sarah Pierce, who tells Nica that the mailman was just being nice and that she doesn't want to see Nica get hurt. They open the package to find a Chucky doll in it. Later that night, Sarah is heard screaming.
After Chucky transfers his soul into Nica, allowing her to walk out of the asylum, she is reunited with Tiffany at the front gate. As they get into her car, Tiffany begins to feel bad about Alice, and says that it was nice to have her around.