On the Harry Potter Frenzy website, I read that the half-blood prince looks like a lion with gray hair and yellowish eyes behind his spectacles (paraphrase). The only chracter that remotely fits that description is Madam Hooch in The Sorcerer's Stone, page 146: "Their teacher, Madam Hooch, arrived. She had short gray hair and yellow eyes like a hawk." What do you think? Could the half-blood prince have transfigured himself into a female Hogwarts gym teacher?