In the second book , Harry Potter and the Chamber of Secrets , Rowling pits Harry against Tom Marvolo Riddle , a memory of Lord Voldemort locked up in a secret diary that Ron 's younger sister Ginny Weasley finds in a bathroom . In the second book , Harry Potter and the Chamber of Secrets , Rowling pits Harry against Tom Marvolo Riddle , Lord Voldemort 's `` memory '' within a secret diary which has possessed Ron 's younger sister Ginny Weasley .