Cecilia

Cecilia was a young, pretty Muggle woman who seems to have been romantically involved with Tom Riddle Sr. prior to his marriage to Merope Gaunt. When Harry Potter viewed the Pensieve showing the heritage of Lord Voldemort, Cecilia was riding on a gray horse past the Gaunt Shack in Little Hangleton with Tom. The two sneered at the ruined cottage, gossiped, and laughed when a terrified Bob Ogden ran from the house and knocked into Tom's horse.

Presumably, Cecilia lost contact with the Riddle family after Tom married Merope, though it is possible that she became part of his life again after he abandoned his wife and returned to Little Hangleton.

Appearances

 * Harry Potter and the Half-Blood Prince