Poison Ivy

Batman: Arkham Asylum

Poison Ivy plays a major role in the plot of Batman: Arkham Asylum. After the Joker and Harley Quinn raid Arkham Asylum, trapping Batman and the guards inside, and releasing most of the inmates, Poison Ivy becomes very concerned for the well-being of the plants in the gardens of the Asylum. When Batman finds Ivy's cell in the penitentiary she urges Batman to release her so that she may save "her" plants, however Batman refuses. Eventually Harley Quinn comes across Ivy's cell and after some discussion between the two Harley releases her.
 
After Batman discovers that the Joker plans to use Titan, a variant of the dangerous chemical Venom, to create his own army of mutants, he visits Ivy in the asylum gardens for assistance in creating a chemical to negate the effects of Titan, due to Venom being made from Ivy's plants to begin with. Batman convinces Ivy that if she does not help him the plants of the asylum will die and Ivy informs Batman that spores for the anti-venom can be found in the lair of Killer Croc. Batman leaves without harming Ivy but threatens her, saying that he'll be back if she does not return to her cell.

The Joker injects Titan into the plant systems Ivy is assimilated with, hoping to create a kind of highly powerful mutant plant, but finds instead he gives Ivy the ability to evolve and grow her plants at an extremely fast rate. Ivy attempts to use her new powers to take over the asylum with her plants and when Batman returns to her it results in a fight between him and Ivy, her plants, and the security guards she has managed to take control of. Batman defeats Ivy, bringing her and her plants under control and eventually defeats the Joker as well, allowing order to be restored in the asylum, at which point Ivy is returned to her cell.

Origins

Poison Ivy's most common origin story is that she was seduced by her professor into committing crimes with him. He later tried to kill her using poisonous herbs. She amazingly developed an immunity to poison and killed her professor. The poison also made her go crazy and become obsessed with plants. She also developed severe mood swings. Her most common weapon is her "kiss of death" where she will kiss her victim with her poison lips. She hates all humans with the exception of Harley Quinn who is her only human friend. Ivy's powers have grown more advanced over the years and she is able to control plants. She has been able to seduce everyone except Batman and this has become her other obsession.