Cassidy is a Team Rocket member of higher rate than Jesse and James, and has appeared in three episodes this far, four totally. To take it simply, her partner is Butch and her Pokèmon are a Raticate, a Drowzee and a Hitmontop. But there is more about the dear Cassidy... so much, much more...
She is just beautiful and evil and powerful and I just don't know what to say... she is an amazing character. In a way she reminds a lot about Jesse; Like her, Cassidy seems to be the boss of her team, she does most of the talking and the battling, she is mean and the one to take action, and she is full of ambitions. That is probably the main difference: Jesse wants to get rich and powerful. Cassidy wants to please her boss. Which she does quite well; According to herself, he bailed her and Butch out of jail himself.
But what really makes Cassidy an intersting character is her relationship with Jesse. Nobody really knows for sure what happened between them, but what is evident is that they knew each other before they met in "The breeding center secret", and that something happened which made them rivals today. In one way one could call Cassidy Jesse's Gary, and in one way it is right; in another not. Jesse absolutely hates her, and Cassidy probably would hated Jesse too if it hadn't been for the fact that she is better than Jesse, she knows it, and she enjoys it for all it's worth. She tells Jesse that Giovanni is growing tired of having to put up with her and James' constand failures and such things, and it is, of course, no wonder why Jesse dislikes her.
But another thing is that nobody knows what happened with them, because they obviously must've been close. When they meet for the first time Jesse claims that "Cassidy used to hang around her because she was jeaouls of Jesse's beauty", and Cassidy says that "You haven't changed a bit". There are some specualtions about this; some thinks they used to be friends and just drifted apart, some that they fought over a boy, and some that they were in love once upon a time.
(Picture is from Together Forever)