Her Pokémon is Paras which evolved into Parasect. She and her grandmother are both experts on medicinal herbs. She scolded Team Rocket for leaving Meowth to fend for himself when he was sick. She gave him some herbs and Meowth fell for her at first sight. She was mad at her grandmother for trying to scare off more customers (Ash and co.) who needed medicine. She suggested that Ash had a Pokémon battle with her, which never went well at all. After a while she explains to Ash and his buddies that she's trying to invent a miracle potion that can do anything and needed Parasect's mushroom to make it happen. After the rematch, and her Paras evolved, she went to work on the potion. At the end she adopted a Persian her grandmother found, unintentionally breaking Meowth's heart.
This Parasect is Cassandra's main Pokémon. When it was a Paras it had a tough time battling because it was so weak, weak enough to be knocked out be even the most puniest of attacks. Meowth came up with a plan to build up confidence in it, by allowing it to win. It was also able to beat Ash's Charmeleon with just one jab to the gut, which gave Paras enough experience to evolve into Parasect. It currently lives with Cassandra making the miracle potion she envisioned.