Ivy first appears as a child and is a character who doesn't do much initially, but a confrontation with a character who can age people to death causes her not to die, but to grow into a woman. She falls into a hole in the ground, which ends up being a drain that sweeps her away.
In Mad City: Burn the Witch... she emerges from the river. Upon making her way to a truck, Ivy sees her reflection in the mirror and notices that she is older and has outgrown her clothes. When a man named Nick comes across her, he offers to take her home to help her out. Ivy introduces herself to him and states that she is thirsty for water. At Nick's house, Ivy drinks some water and then pours the rest on a plant. While stating her history and how she "grew," Ivy was displeased that Nick threw out one of his plants. Later that night, Ivy changes into the clothes of Nick's ex-wife and drops a flowerpot on Nick's head, killing him. Before leaving Nick's corpse, she quotes "Next time, remember to water.". She murdered a man who had helped her purely because he threw a plant out.
In Mad City: The Executioner Ivy seduces a rich man into opening a vault full of riches by controlling him with a perfume. She tricks the man into smelling the perfurm on her neck which places him into a controlled state for Ivy to use as she wishes. As he obediently opens the vault, Ivy grabs a vase and smashes the man over the head with it. She enters the vault and steals a necklace, as she is escaping the man recovers from his floored postion and grabs Ivy but she knees him in the balls and runs away.
In A Dark Knight: Pieces of a Broken Mirror, Ivy now has powers much like the Poison Ivy found in most DC Comics work, and the mutation has caused her to look like a different person as well (Peyton List begins the role of Ivy from this episode onwards).
The character of Ivy Pepper is based on the DC Comics villainess Poison Ivy aka Dr. Pamela Isley, and may just be the same character with a slightly different name. She has already shown that she shares Poison Ivy's love for plants.