They first appeared in "Kindred Spirits". Vlad wanted the perfect half ghost son, but instead he got several failed ghost clones and a daughter, Danielle. Besides her, the clones were a Bed Sheet Ghost (revealed to be a skeleton when uncovered), a muscular and tall monster-like ghost, a tiny ghosts who later revealed himself to be a deformed version of Danny, and a prime clone. All of the clones, except for Dani, dissolved in to ectoplasm. They all served Vlad as if he were their father, but Dani later realized she was just "a mess that Vlad's not going to clean up". She teamed up with Danny to defeat Vlad and left afterwards.
Dani reappears in "D-Stabilized". She is running away from Vlad, and is searching for Danny so he could stabilize her, but both of them end up being caught by the mayor's assistant, Valerie Gray. Vlad was attempting to catch and melt Dani so that he could study her ectoplasmic remains in order to make a perfect clone, but fails, thanks to Danny and Valerie, who thought that Vlad Plasmius was the villain and Vlad Masters was a victim. Danny managed to stabilize her with the "Ecto-Dejecto", and she once again flew away.
Even though most of the clones vanished in their first appearance, several of them (including Dani herself) can be seen in "Phantom Planet".
List of Clones