Severus Snape
Voldemort is a half-blood, and his most guarded secret which few Wizards know is that his father was Tom Riddle, a Muggle. Severus Snape is also a half-blood (he gave himself the nickname "The Half-Blood Prince"), as his father Tobias Snape was a Muggle. Harry himself is a half-blood, since his pure-blood father, James, married a Muggle-born witch named Lily, and his maternal grandparents were Muggles.