They can bury me in the ground, as deep as they like. But I'll grow back. We always grow back. Don't we, baby?
~ Poison Ivy talking to one of her plants
Poison Ivy (real name: Dr. Pamela Lillian Isley) is a major recurring antagonist in the DC Animated Universe. She is a botanist who led a secret life as a eco-terrorist. She is Harley Quinn's best friend and roommate.
Poison Ivy is a major antagonist in Batman: The Animated Series and its sequel The New Batman Adventures and one of the two main protagonists of the Gotham Girls spinoff series.
In her early life, Pamela Isley had a PhD in botany from Gotham University, She was once employed by Chez Gerard, a cosmetics firm, as a research chemist in charge of developing new perfumes. She also offered a weekly lecture series at the university on rare and extinct plant species. At some point, she became the meta-human known as Poison Ivy and who is now part-plant, able to communicate telepathically with plant life, and capable of synthesizing chemicals from her body at will, either as deadly poisons or antidotes.
Poison Ivy's first act was manipulating District Attorney Harvey Dent into having a date with her, because she had marked him as a "murderer" for his part in arranging the excavation of land for the future site of Stonegate Penitentiary. The excavation had involved in bulldozing a field of wildflowers, including a rare wild rose that would have gone extinct if she had not saved it first. Appropriately, she used extract from the rose to create a rare and deadly poison, which she administered to Dent through a deep kiss. He fell into a coma and was not expected to live.
However, Dent's personal friend Bruce Wayne (Batman) investigated the crime and discovered the origin of the poison, Batman confronted Isley, who now calls herself "Poison Ivy". In a ensuing struggle, Batman captured the rose as a "hostage", forcing Ivy to surrender the antidote she had developed. He managed to save Harvey's life, and Ivy was imprisoned in Stonegate. She was later transferred to Arkham.
A few months later, Ivy may have escaped or been released from Arkham, she opens an all-natural resort spa outside of Gotham, under the alias "Dr. Daphne Demeter". The "Eternal Youth Spa" was a bait through which Ivy attracted wealthy business men and women guilty of certain environmental crimes, and "treated" them with plant-derived toxins, which transformed the people into plants themselves.
By unfortunate coincidence, a greedy Wayne Enterprises executive had made a deal with a South American company to level a section of the Amazon rain forest, before Bruce aborted it. Ivy targeted Bruce for his perceived part in the deal, and Alfred Pennyworth attended in Bruce's place for a free spa getaway. When Alfred disappeared, Batman quickly deduced the doctor's true identity, and stopped Ivy's plan yet again.
Sometime later, during a heist at the Gotham Museum, Ivy meets Harley Quinn for the first time, who was recently evicted from the Joker's gang, and she helped Ivy escape the clutches of the Gotham Police Department. Though they seem to have completely opposite personalities, Ivy and Harley have (somewhat) common goals bonded them into a formidable duo. Harley seemed to thrive on the experience of an honestly affectionate partner, while Ivy admired Harley's streak of fun and ingenuity, though she was exasperated by Harley's unshakable devotion to the Joker. The girls become the very best of friends and sister-in-arms.
Shortly after uniting, they took Gotham by storm with a crime spree. Dubbed "The New Queens of Crime", their actions went unstopped for a long time. Despite Batman's best efforts, he was unable to capture the two girls. It was only after when their home was destroyed by the Joker, that Ivy and Harley were captured by Renee Montoya.
Poison Ivy sees Harley Quinn as a little sister; she will do anything to protect her. Ivy also seems to love Harley more than anything else in the world, despite her Misanthropic nature and undying love for plants.
Ivy's fate in the TNBA episode Chemistry is similar to that of Mr. Freeze in the full-length movie Subzero. Their ocean properites (Freeze's oil rig, Ivy's boat) catch fire and explode, and Batman leaves them to drown. The only difference is that Freeze survives and swims to shore with his polar bears, whereas Ivy appears to drown.