Secret-Keeper

Your hero has a dirty little secret that is central to the character and the show's main plot. They might be a superhero, or he might have a lab in his bedroom, or she might be a witch. There will sometimes be one regular Joe who knows about the hero's secret, and will also keep it secret. The Secret Keeper can be family, though it's very uncommon for them to be parents. It may require a Secret Relationship, if public knowledge of the relationship is a danger to the secret.
Often, the existence of a Secret Keeper is only hinted at, rather than explicitly revealed to either the hero or the audience.
The phrase originated in Harry Potter, where the Fidelius Charm hides a secret within a living person's soul (that person would then become the "Secret Keeper"); everyone else in the world other than the chosen person would be unable to learn the secret unless directly told of it by the Secret Keeper. Even if they originally knew the secret, the details would become muddy in their minds unless reminded by the Keeper.
See also Tongue-Tied, for when a character is forced to keep a secret. See also The Confidant, someone who keeps non-heroic secrets. For cases in which the hero doesn't know that someone knows their secret, see Secret Secret-Keeper. Frequently discovers Keeping Secrets Sucks, and is witness to The Masquerade Will Kill Your Dating Life.
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   Film — Animated 
* Ratatouille: Ego wrote his review without mentioning that "the genius now cooking at Gusteau's" is a rat, in order to protect the employment of "the finest chef in France," but the Health Inspector and Skinner shut down the restaurant because of rats either way. That said, he does foot the bill for a new restaurant officially run by Linguini and Colette, and, for obvious reasons, he's more than willing to keep the secret afterwards.
   Film — Live-Action / Literature 
* Harry Potter:
** As stated above, the trope namer is the Fidelius Charm, a spell that makes one individual a literal Secret Keeper; as long as they keep the secret, it cannot be found out by anybody else. In addition, even if the Secret Keeper does tell someone the secret, that someone is unable to pass on the knowledge unless the original Secret Keeper dies, in which case they become the new Secret Keeper. If the Secret Keeper dies without passing the knowledge to anyone else, then the secret will remain secret forever. This is most often used to conceal locations.
*** The house where Harry and his parents lived was guarded under the Fidelius charm so that Voldemort couldn't ever find them. As pointed out by Professor Flitwick, Voldemort could stumble on the Potters house, but without being told by a Secret Keeper, he would never be able to spot them. They specifically chose Peter Pettigrew as Secret Keeper, because everyone would naturally assume James Potter would select his brother-from-another-mother best friend, Sirius Black. Knowing that, the Potters decided, at Sirius's suggestion, to choose a less obvious Secret Keeper and Pettigrew became it. They just didn't know that Peter was a Death Eater and loyal to Voldemort. During his angry rant in Prisoner of Azkaban, Sirius muses that if Peter chose to die without telling Voldemort, then the Potters would have been safe forever.
** Alan Rickman became a Real Life Secret Keeper about Severus Snape's motives, when J. K. Rowling told him that Snape loved Lily Potter years before this information was revealed in the books.
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