Cassidy Gamp is a Hufflepuff at Hogwarts, with yellow hair and blue eyes. She dated Charlie Plex, but they broke up when Charlie informed her that he'd been sleeping with Donna Shacklebolt. Upon learning this, Cassidy hexed Donna from behind, landing the latter in the hospital wing but not badly hurt.
At the beginning of seventh year Cassidy and Donna met again on the train to Hogwarts. Cassidy greeted her harshly, but Donna ended up apologizing for what she'd done with Charlie. Cassidy was pleasantly surprised to discover that the Donna Shacklebolt actually felt guilty.