Claudia Sloves is two years older than her twelve-year-old brother, Jason. Ever since they were young, Claudia has tormented Jason by pranking and teasing him. Some of her jokes are tame, like putting ice cold hands on Jason's neck. However, some of her jokes are more extreme, such as the instance in which she tied a dead rat to fishing line and pulled it to trick a ten-year-old Jason into believing it was alive.
Throughout the book, Claudia continues to prank Jason. She eventually writes a story about how the mirror is haunted, and — at one point — she wears a rubber monster mask and jumps out at Jason. When Jason finds a threatening note beside his new mirror and dresser, he appropriately assumes its one of his sister's tricks. However, it turns out that mirror is sinister, and Jason ultimately has to destroy it.
Claudia has an incredibly bold personality. At school, this works in her favor, as she's popular and has a large friend-group. However, her sense of humor is twisted, and she ends up treating her brother very poorly for her own amusement.
Claudia has thick, springy, black hair and wears red glasses. Additionally, she has red-and-blue braces, which are often bound with rubber bands.