Mirielle and her brother Sy Borgman were both in the CIA. Sy was their "go-to cleaner", while Mirielle was a hybridization scientist who specialized in grafting animals together. To help with his missions, Sy asked Mirielle to create a hybrid of a monkey and octopus (a Monkeypuss), and although she claimed it was against protocol, she eventually made the attempt because she loved him. While she was securing a monkey and octopus in a chamber for merging, the monkey escaped and hit the button, trapping her inside with the octopus and transforming her into a giant tentacle monster, After that Sy locked the tentacle monster in the basement of the Gotham Mall for 35 years, during that time she ate livestock that Sy gave her.
In "L.O.D.R.S.V.P." one of the current occupants of Gotham Mall, Doctor Psycho became suspicious at the titanic rumbling reverberated throughout the mall. Sy tried to deny he even heard it, but when it continued he then tried to assert that it was "just the pipes", at which point a pink, fleshy tentacle erupted from the ground, grabbed Doctor Psycho, and smashed him against the ground and ceiling repeatedly. Sy finally admitted that there "may be a giant tentacle monster in the basement" and explained that it was his sister, telling him the story of how she ended up that was. . Doctor Psycho wanted to kill "that noise making abomination" immediately, even arming himself with a harpoon, but Sy insisted that it wasn't an abomination and was in fact his own sister.
Sy then took Psycho to Mirielle and Psycho used his telepathic powers to restrain her when Harley Quinn, Clayface and King Shark suddenly fell through the floor. Doctor Psycho filled Harley in as Sy continued to sob with guilt, and King Shark suggested that Sy should talk to Mirielle. Doctor Psycho decided to "be your Whoopi" and translate telepathically between Mirielle's thoughts and Sy Borgman's words. Sy apologizes for Mirielle's predicament, but she claimed that it was not being transformed into a tentacle monster that angered her, but rather the decades of abandonment and of Sy closing himself off to her that had made her angry.
Psycho continues translating, saying "I always thought no matter how bad it got, you'd never give up on our friendship." This prompts Mirielle and Harley to begin crying as well, and Sy says he should have come to Mirielle, but feared that she wouldn't forgive him. At this point, Mirielle then picks Sy up with a tentacle and hugs him gently. Overjoyed, Sy claims he needs no translation for the hug. He says he never should have kept Mirielle trapped in his basement, and promptly frees her. Mirielle joyfully surges out of the basement and into Gotham, with Harley tearfully saying she looks "great" and seems so happy. However, Mirielle immediately begins destroying helicopters, attacking cars, and eating civilians.