Who is the half-blooded prince?

Professor Severus Snape

He is a half-blood because his father, Tobias Snape, is a muggle, but his mother, Eileen Prince, was a pure-blood witch. Due to his Death Eater interest, he clung to his mother's heritage and used her maiden name to create his pseudonym. Snape revealed he was the Half-Blood Prince as he was fleeing from the castle after killing Dumbledore.