Dr. Pamela Isley, also known as Poison Ivy is a criminal and a patient at Arkham Asylum.
In Gotham City, the yellow rings of Parallax head to Arkham Asylum. There, Batman and Nightwing are doing their best to contain the newly powered inmates of Arkham, including Poison Ivy, who have already been transformed into Yellow Lanterns. Luckily, Superman arrives just in time to offer assistance to Batman and Nightwing.
Batman and Nightwing find it difficult to defeat the super-powered inmates, as Poison Ivy has captured them in her vines. After been attacked by Bane, Poison Ivy and Mr. Freeze, Batman tells Superman that the ring is not doing either of them any bit of good. After this, the Yellow Lanterns stand still and kneel before him. Parallax suddenly appears behind Superman, having John Stewart under his control.
Emil Hamilton succeeds into finding a way to reboot the rings so when he does all the Yellow Lanterns, including Poison Ivy, are released from the influence of fear and lose their powers with the rings turning black.
After their rings get rebooted and they lose their powers, the inmates of Arkham fall from the sky unable to do anything to escape from their eventual death but fortunately Superman manages to save them all.
After Parallax's defeat, all the now depowered criminals return to Arkham Asylum.
In the Comics
Pre-Crisis, Poison Ivy's real name was Dr. Lillian Rose, a promising botanist from Seattle. She was seduced by Marc LeGrande into assisting him with the theft of an Egyptian artifact containing ancient herbs. Fearing she would implicate him in the theft, he attempts to poison her with the herbs, which are deadly and untraceable. She survives this murder attempt and discovers she has acquired an immunity to all natural toxins and diseases. That was the birth of Poison Ivy.
Following the events of Crisis on Infinite Earths, which massively retconned DC Universe history and continuity, Poison Ivy's origins were revised in Secret Origins #36. Pamela Lillian Isley grows up wealthy with emotionally distant parents. She later studies advanced botanical biochemistry at a university with Alec Holland under Dr. Jason Woodrue. Isley, a timid shy girl, is easily seduced by her professor. Woodrue injects Isley with poisons and toxins as an experiment, causing her transformation. She nearly dies twice as a result of these poisonings, driving her insane. Later, Woodrue flees from the authorities leaving Isley in the hospital for six months. Enraged at the betrayal, she suffers from violent mood swings, being sweet one moment and evil the next. When her boyfriend has a car accident after mysteriously suffering from a massive fungal overgrowth, Isley drops out of school and leaves Seattle, eventually settling in Gotham City.
She begins her criminal career by threatening to release her suffocating spores into the air unless the city meets her demands. Batman, who appears in Gotham that very same year, thwarts her scheme, as well as she is incarcerated in Arkham Asylum. From this point on, she has a kind of obsession with Batman, he being the only person she could not control. Over the years, she develops plant-like superpowers, the most noticeable being a lethal toxin in her lips: she is able to literally kill with a kiss. While in Arkham, Poison Ivy receives a message through flowers that someone is to help her escape. That night, two women, successfully break Ivy out and bring her back to their employer. She is less than happy to discover that it is the Floronic Man, formerly known as Dr. Jason Woodrue, her former college professor that conducted the experiments on her. After striking a deal with him in the underground tunnels of Gotham, Ivy receives a trunk full of money in return for samples of her DNA. Woodrue intends to combine their DNA to create a "child", all while flooding the streets of Gotham with high-powered marijuana. The purpose of this is to create a world economy run on hemp and to have their offspring control it. Batman intervenes, but is overcome by Woodrue's henchwomen. However, Ivy turns on Floronic Man and lets Batman go to fight the intoxicated maniac. In the end, Batman decapitates the Floronic Man and Ivy escapes with her money.
When Gotham City is destroyed in an earthquake, rather than fight over territory like most of Batman's enemies, she holds dominion over Robinson Park and turns it into a tropical paradise. Sixteen children who are orphaned during the quake come to live with her as she sympathizes with them having suffered a traumatic childhood herself. That winter, Clayface pays Ivy a visit, hoping to form a bargain with her. This would entail her growing fruits and vegetables, having the orphans harvest them and him selling the produce to the highest bidder. She wants nothing to do with the plan and she attempts to kill him with a kiss. Clayface overpowers her, however, as well as imprisons Ivy and the orphans for six months in a chamber under the park's lake. He feeds her salt and keeps her from the sun to weaken her. Eventually, Batman comes and discovers the imprisoned orphans and Ivy. The two agree to work together to take Karlo down. Ivy fights Karlo, ensnaring him in the branches of a tree and fatally kissing him. She then proceeds to sink him down into the ground, where he becomes fertilizer for her plants. Batman, originally intending to take the orphans away from Ivy, recognizes that staying with her is what is best for them and they remain in her care until the city is restored. Also, as part of a bargain to keep her freedom, Batman arranges it so that Ivy provides fresh produce to the starving hordes of earthquake survivors. Soon after, Ivy finds Harley Quinn, who had almost been murdered by the Joker, among the debris of the earthquake and nurses her back to health. The two have been best friends and partners-in-crime ever since.
After Gotham City is reopened to the public, the city council wants to evict her from the park and send her back to Arkham Asylum, as they are uncomfortable with the thought of a "psychotic eco-terrorist controlling the equivalent of 30-odd square blocks." They also mistakenly believe that the orphans in Ivy's care are hostages. The Gotham City Police Department threaten to spray the park with a powerful herbicide that most certainly would have killed every living plant in the park, including Ivy and more than likely do harm to the children. Ivy refuses to leave the park to the city and let them destroy the paradise she had created, so she chooses martyrdom. It is only after one of the orphans, is accidentally poisoned by Ivy that the hardened eco-terrorist surrenders herself to the authorities in order to save the girl's life. Batman says that, as much as she would hate to admit it, Ivy is still more human than plant.
In Batman: Hush, she and other Gotham criminals are manipulated by the Riddler and Hush. Her task is to hypnotize both Superman and Catwoman, using Catwoman to steal ransom money from Killer Croc after the original plan is interrupted by Batman while Superman serves as her "bodyguard" when she hides in Metropolis. However, she abandons Catwoman to be killed by Killer Croc and Batman is able to make Superman break out of the spell. Soon afterwards, the Riddler approaches Ivy and seeks her protection. Ivy, who is angered by the manipulation, battles the Riddler physically and psychologically. She comes to physically dominate her opponent, humiliating Riddler and temporarily breaking his spirit.
In One Year Later, Ivy's control over flora has increased, referred to as being on a par with Swamp Thing or Floronic Man. She also appears to have resumed her crusade against the corporate enemies of the environment with a new fanaticism, regarding Batman no longer as a main opponent, but as a "hindrance." After arriving back from a year-long absence, Batman discovers that Ivy has been feeding people including "tiresome lovers", "incompetent henchmen" and those who "returned her smile" to a giant plant which would digest the victims slowly and painfully. She refers to these murders as a "guilty pleasure". In an unprecedented event, her victims' souls merge with the plant, creating a botanical monster called Harvest, who seeks revenge upon Poison Ivy. With the intervention of Batman however, she is saved but she is left in critical condition.
In the Battle For The Cowl storyline, she is coerced by a new Black Mask into joining his group of villains that aims to take over Gotham. Shortly after, she escapes from Black Mask's control and forms an alliance with Catwoman and Harley Quinn, leading into the ongoing series Gotham City Sirens. When Harley Quinn betrays her friends and breaks into Arkham Asylum orchestrating a violent takeover of the facility, along with the Joker, Poison Ivy arrives and tries to convince Harley that the Joker is evil, but Harley refuses to believe her and knocks Ivy unconscious. After they are defeated by Catwoman and Batman, Catwoman then tells Poison Ivy that they are no longer friends, after Ivy had drugged Catwoman in an attempt to discover Batman's identity. Then Ivy is taken in Arkham Asylum. Ivy soon escapes and ambushes Harley in her cell, binding and gagging her former friend before she can defend herself. Ivy struggles with the decision to execute Harley for her betrayal, but ultimately releases her after realizing that she is still her friend. Together, the two set off to find Catwoman and make her pay for leaving them behind. The two of them found Catwoman and fought her on the streets. While they were fighting, Catwoman confessed that she saw good in the both of them and only wanted to help them. When she told them that she only kept tabs on them because Batman wanted to keep them under control, Ivy lashed out onto the city by using giant vines to destroy buildings, cursing at Batman for manipulating her. Batman was about to arrest them, but Catwoman helped the two of them escape.
In the New 52, Pamela Isley was born with a skin condition that prevented her from leaving her home, the garden was the place where she spent most of her limited time in the outside. Her father constantly beat her mother until he finally murdered her and buried her in the garden. While in college, Pamela sold pheromone pills to other students to study its effects until she was caught by police. She used a powerful version of the pills to mindcontrol the Dean so he would drop the charges and let her graduate from college with honors. While visiting her father in prison, she kissed him with a poison that was secreted from her lips that killed him. Later she landed an internship in Wayne Enterprises in its Bio-Chem division developing pharmaceutical and cosmetic applications. She was fired after proposing Bruce Wayne to develop chemicals that trigger social or behavioral responses. As she was escorted out by security, she accidentally spilled the chemicals she was working with on herself, giving her powers to control plant life and immunity to all poisons and viruses.
Poison Ivy is recruited into the covert-ops group known as the Birds of Prey. Though she is specifically hand-picked by the team's leader Black Canary, the other members of the group protest Ivy's inclusion, citing her violent past and connections to various murders. These suspicions are proven correct when Ivy poisons the team and forces them to attack corrupt companies she wants to destroy until Katana apparently kills her. Ivy survives the injuries and returns to Gotham, breaking out Basil Karlo in order to manipulate him into becoming her husband. Batman intervenes to help her. Ivy is later captured by Penguin's men, as the locations she attacked were the his properties. She is buried alive by them, but survives long enough to be rescued by Penguin's right-hand man, Ogilvy, who has taken his boss' businesses after the Joker's return. He proposes an alliance with her. However, Karlo, who Batman had set free from Ivy's control, tracks down and attacks Ivy, with her fate being left unknown.