* Professor Lupin, the Defence Against the Dark Arts teacher, who has the secret of being a werewolf. This is a major plot point in the book.
* Professor Snape, the Potions Master, who has many secrets including his love for Lily Potter and his animosity towards James Potter. While he doesn't have one specific "secret," his motivations are a mystery throughout the book.
Could you please clarify who you're referring to? I'd be happy to provide more information!