Maggie Kyle is the younger sister of Selina Kyle, the Catwoman. Unlike her villainous sister, Maggie is a nun. She eventually left the convent. Maggie's relationship with Selina grew distant as they got older. She eventually left to find a new life and explore the world. Moving to the West Coast, she met her future husband, Simon Burton. Years later, Simon's work transferred them to Gotham City, where Maggie reunited with her long lost sister, Selina.
The reunion was short lived though when she and her husband Simon were kidnapped by Black Mask all in an effort to hurt/get to Catwoman. Though she was rescued by Selina, it was too late. She had been tortured and forced to eat her husbands eyes. Simon died at the hands of Black Mask. The events caused her to snap. She was then entered into a psychiatric institute.
She remained in there for some time, before the Black Mask returned from the dead in Blackest Night, seeking revenge on Catwoman for killing him. The fear of seeing him again set Maggie to new lows and even though her sister was helping her, she blamed Selina for everything that had happened to her. As Selina and the Gotham City Sirens fought Black Mask, Maggie ran away.
Gotham City Sirens
Catwoman gets information on her escaped, mentally unstable sister Maggie while she was on another private assignement. Maggie believed that her sister was possessed by a demon since the sign of the demon is a the cat. Maggie sought out Sister Agatha, a renowned exorcist, to help her save Selina but while Maggie was in Agatha's apartment, Maggie lost control when she saw the Agatha's cat. Maggie killed the cat and killed Agatha because she assumed Agatha was in league with Selina's cat demon.
Maggie searched through Agatha's apartment and found many relics as well as tools of the exorcist's trade. Maggie steals Agatha's exorcism equipment but she also comes across something bizarre amongst Agatha's weapons and crucifixes. Maggie discovers a container with a supernatural substance in the guise of an angel. The vile broke on the floor and the substance gave Maggie supernatural strength, speed, stamina and agility. It also gave Maggie a warped sense of reality because the vile releases what Maggie believed to be an angel. The angel claimed it would guide Maggie and help her destroy the cat demon inside her sister's soul but only by allowing the madness to enter Maggie's mind.
Maggie finds her sister along with Harley Quinn and demonstrates that she is more than a match for the two Sirens. After calling Maggie "Sister Zero", Harley becomes possessed by the angel's influence. Fortunately, Catwoman is able to get through to Harley and confuse her enough to escape Maggie's control. Selina then forces Maggie to escape and Maggie runs away only to create a new plan to save her sister's soul.
The angel that appeared to Maggie told her to team up with Azrael and Crusader during Gotham's judgment day. Crusader interrogates Maggie in order to see if she is worthy to join Azrael's cause. Azrael tells Crusader to stand down and allow Maggie to present herself. Maggie tells Azrael that she believes in his crusade against the evil in Gotham City but her role during Gotham's destruction is narrowly single.
Maggie believes she is her sister's test of faith which will either cleanse her sister's soul of the cat demon that has supposedly possessed her or Maggie will die trying. In the end, Maggie has a chance to end Selina's life but she couldn't bring herself to do it. Azrael approaches Maggie after she let Selina go and he asks her if she will join his crusade despite her failure. Maggie tells Azrael that she will not join him at this time but in the future who knows.