David Cain was the loyal assassin to "Mother". While utterly devoted to her and her ways, when Mother began using Doctor Jonathan Crane's Fear Gas to simulate trauma, David felt that the old ways of "molding" a child through the trauma of witnessing actual death was still viable and sought to prove this to her by secretly siring and molding a child of his own to serve her as a tribute.
Following a theory of his, Cain kept the child; whom he named "Cassandra", at The Nursery and kept her from interacting with anyone other than him, never allowing her access to verbal communication. This forced the parts of her brain that were normally used for speech to be trained to instead read other people's movements and body language and by extension predict their next move, much like how a person might predict how a sentence would be structured by watching it being created.
Subjecting her to witness numerous killings and slaughters, Cain than forced her to fight various opponents, all of whom she bested. However, when Cain revealed his experiment to Mother, he was berated for his reliance on the old ways and, although he was allowed to keep her, she warned him not to let her see "his pet" again.
Some time later, on September 29th, Mother decided to have David use Cassandra to assassinate the parents of a child named Harper Row as part of a ploy to force Batman (Bruce Wayne) into accepting a "perfect soldier" from her to use as his next Robin in his war on crime. While Batman fell for the ploy, despite his best attempts not to, Cassandra was unable to carry through with the assassination; only managing to kill the mother, Miranda Row, before emotionally breaking down.
Leaving Miranda's body at her family's doorstep (while her husband, Marcus Row, had abandoned her to be assassinated) Cassandra returned to her father as a failure.
Some years later, when Mother intended to return from hiding and hypnotize the planet's children into killing all adults, Cassandra sought Batman's help, only for him to direct her to approach Richard Grayson instead as he was on the verge of facing the Joker at the pinnacle of the villain's "Endgame" attack.