Cassandra was originally never supposed to be born, for both her parents are vamponies. She was created with the help of Al's alchemy.
In episode 10 of Season 1, Cassandra was shown as Marisa's daughter in the future. When she was first release in the Present time, Dempsey, Marisa's father, had to somehow find a way to bring Heart back into the future before Future Marisa had found out and had ruined the entire time period. Heart was successfully brought back in time by Dempsey and now remains there... Hopefully. It is so far not certain that Cassandra will once again appear in future episodes, but there has been speculations.