Your hero has a dirty little secret that is central to the character and the show's main plot. He 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.
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 his 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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Sonic X has Chris Thorndyke, his grandfather, and his two servants Mr. Tanaka and Ella right before everyone knows about Sonic and his friends.
The whole premise of Fruits Basket is Tohru Honda serving as the Secret Keeper of the Sohma family if she wants to continue living with them, leaving even her two closest friends out of the loop — even though one of them has Electric Psychic Powers.
Melchisedec and Carmichael in Soukou No Strain serve as Sara's Secret Keepers despite not knowing just what her secret is, only that she can synchronize with a discarded Mimic.
In the classic anime Kimagure Orange Road, blood members of the Kasuga family all had psychic powers and a desire to keep them a secret from the general populace. This necessitates that anyone who marries into the family (such as Kyousuke's father) must become a Secret Keeper. The manga takes this to its logical conclusion that members of the Kasuga clan must reveal their powers to those they intend to marry before they can get hitched. Hopefully, if you love them enough to marry them, they like you enough not to blab your secret to the world.
The other major secret that Kyousuke has - his budding love for Ayukawa Madoka - is kept by his sister Manami. In the manga, she even explains it to Hikaru, with the iconic "red hat" as proof.
The Master of the local coffee shop, Abcb, knows the feelings that both Kyousuke and Madoka have for each other.
Misa, Mikami and Takada all knew Light's secret at some point and helped him in his schemes in Death Note. Ryuk and Rem knew it too, but as invisible shinigami, most of the time they weren't even able to tell anyone if they wanted to.
Dr. Agasa, Yusaku and Yukiko Kudo, Phantom Thief Kid, Heiji Hattori, Ai Haibara, Vermouth, and Eisuke Hondo in Detective Conan.
Well, actually Ai Haibara and Vermouth are part of the secret. In fact, it could be argued that they are directly responsible for it, so they may not qualify
The trope gets invoked in a nasty manner in a filler case. A girl named Shouko Utakura finds her friend Sawanami dead and, due to his dying clues, she realises that their common acquintance Mifune is the murderer. However, Shouko is Mifune's Unrequited Tragic Maiden and decides to keep this a secret... but commits the huge mistake of actually telling Mifune that she knew the truth, but wouldn't snitch. Mifune, who had killed Sawanami because he was involved in his girlfriend's death and now believes Shouko had something to do with it too, thought she would denounce him anyway and few days later he strangles her to death.
Rin Sawamura, Shiro and Nao Makinoha, Shuichi Takamizawa, Kota Shingyoji, Hisashi Sakisaka, and Lucy Winladd in Midori Days.
The Tendo family, Ryoga, Shampoo and her grandmother, Mousse, Ukyo, Konatsu, and Nodoka in Ranma ½.
This is only in the anime - in the manga, it's easier to list those who don't know Ranma's "sercret."
Kagome's mother, brother and grandfather keep Kagome's time-travelling a secret. This leaves Souta to pick up the slack for Kagome at school (ensuring she knows when exams are and what homework she needs to do) and her grandfather creating hilarious and embarrassing medical conditions to both excuse her from school and prevent her friends from visiting her.
Fresh Pretty Cure! has Kaoru, Miyuki, and later in Episode 45, the trio's parents and friends.
Heartcatch Precure has Tsubomi's grandmother, Kaoruko. It also helps that she was a former Cure herself.
The Pretty Cure All Stars movies reveal that each of the teams know of each other's identities. Except for the new team, which quickly becomes moot after awhile.
In DokiDoki! Precure, it would seem that Sebastian is the only human who would know the Cures' true identities. That is until in episode 41 that Alice's friend/rival, Reina, becomes one. If that wasn't enough, the Cures' family and friends got to chance to find out when Cure Heart ended up revealing who she is.
Himechan No Ribon has Daichi becomes the secret keeper for Hime-chan about her ability to magically transform after she had to do it in front of him to save somebody.
Karin has revealed herself as Buurin to her classmates, Kouichi, Masami, Kashiwagi, Kaoru, Jimmy, Kyoko, and Takuma in Episode 50.
Both Isshin and Karin are aware of Ichigo's activities as a shinigami, but don't let on.
Tatsuki, Keigo, Mizuiro and Chizuru are also aware that something is happening but tell nobody outside their circle. Eventually, Ichigo admits everything to them.
Kon became a reluctant secret keeper for Isshin. He found out hundreds of chapters before Ichigo that Isshin was a shinigami, but Urahara and Isshin both swore him to secrecy.
Yumichika, Renji, Keigo, Komamura, Iba, Akon, Mayuri and Ichigo are all secret-keepers for Ikkaku's bankai. Mayuri's a subversion. He accidentally told Ichigo before remembering it's a secret, and doesn't care he revealed it. Ikkaku doesn't know Ichigo knows.
Hisagi is the sole living person who knows the truth about Yumichika's abilities.
The Ishida family is such an unknown to the Vandenreich that Yhwach used Exact Words to lie with the truth, ensuring the Stern Ritter learned only of Uryuu Ishida's existence. While it's implied Haschwalth knows about the family, it's confirmed that Quilge Opie definitely knew all about Souken and Uryuu Ishida, and the mysterious Ishida family secret.
The whole Host Club keeps Haruhi's real gender a secret since she's working as a Host to pay back her debt.
Ito and Akane in W Juliet guard the secret of Makoto's real gender though a few other people eventual figure it out they have much more passive rolls.
In Genshiken it is implied that Ohno knows about Madarame's crush on Saki, as she tries to covertly give him pictures of Saki in cosplay outfits.
Sena's title Eyeshield 21 is kept by Hiruma, Kurita, Panther, Shin, the Ha-Ha Bros.
In Code Geass, C.C. is a Secret Keeper (and sometimes Body Double) for Lelouch. Euphemia is a Secret Secret Keeper, since not even Lelouch realizes that she figured out on her own that Lelouch was Zero. Lelouch gains many more Secret Keepers as the second season progresses with the likes of Kallen and his own maid, Sayoko, but after the Grand Finale, only a handful of people in the world know that Zero has been replaced by Suzaku Kururugi.
A case shows up in One Piece, regarding what really happened between Zoro and Kuma. Aside from Zoro himself, only three members of the crew know the truth: Brook, who saw it happen (since it's next to impossible to tell if a skeleton is conscious or not); Sanji, who heard it from the Risky Brothers and told them not to tell anyone else; and Robin, who used her powers to eavesdrop on that conversation. And Zoro only knows that Brook knows; the other two keep the knowledge to themselves because it would devastate Luffy if he found out, and Zoro didn't do it to be praised anyway.
Also, the Gorgon Sisters. They would sooner die than let anyone find out that they were ever slaves to the World Nobles; only four known individuals have been trusted with the secret. The first three are Silvers Rayleigh, Shakky, and their surrogate mother Elder Nyon, who helped them get back to Amazon Lily after they escaped. And the fourth is Luffy, Hancock's love interest.
A handful of one off characters in the Sailor Moon anime find out about one or more of the Sailor Senshi's identities, but they usually only show up in an episode each. The manga has a few more ongoing characters learn of the masquerade (most notably Motoki). It is implied in both during R/the Black Moon arc that Naru does know Usagi is part of something unusual and important but that Usagi doesn't want her to ask about it, so she complies to not put pressure on her.
In Karin, the character KentaUsui ends up pretty much strong-armed into becoming one for the Marker/Maaka family when he learns that their middle daughter Karin is a vampire.
In Baccano!, Ronnie Schiatto serves as Maiza's secret keeper and only constant companion up until everyone in the Martillo family winds up immortal, and Maiza catches on that that Ronnie is a lot less human than he originally presented himself as, at which point the roles reverse.
In Bakuman。, Moritaka Mashiro, after finding out that his fiancee Miho Azuki's mother Miyuki once loved his uncle, Nobuhiro, and that Miho is unaware of this, asks Miyuki not to tell her. She agrees and asks Moritaka's partner Takagi to do the same. This comes back to haunt Moritaka later, as Miho becomes angry with him for keeping it from her, and does not believe they should have secrets between them.
In Mai-HiME, Takumi becomes one for Akira when he first finds out that Akira is the "secret ninja" protecting the school, and later when he finds out that Akira is female.
In Sakurasou no Pet na Kanojo, Sorata is asked to keep one from Misaki regarding Jin. He finds out Jin wants to go to a university in Osaka, not a local one nearby where Misaki was going. Jin says he's going to tell her at some point, and asks him not to tell her about it.
Then Misaki finds out about it anyway in episode 11, although it was Fuuka, Misaki's sister, casually mentioning it to Jin, who says she heard it from his mother. Although Sorata comes clean about knowing the secret, Misaki seems more excited by the fact that he turned down her sister, which mean she still has a shot with him.
In Sekirei, a majority of the Single Numbers (with the exceptions of Tsukiumi and Akitsu), as well as Uzume, Seo, and the twins Hikari and Hibiki, are sworn to secrecy about the identity of the mysterious #01. It is Harem Nanny Miya Asama. The secret is kept primarily because they are terrified of what she will do to them if they let it slip. Even the Big Bad honors the secret, and leaves her alone.
In Tonari no Kashiwagi-san Yuuto finds out about Kotone secretly being an otaku, which is what kicks off the plot. Sayaka, a Childhood Friend of Kotone, was already in the loop, and eventually more characters find out. Tina figures it out herself almost immediately when she notices the phone strap Kotone has, and Yuuto tells her to keep it a secret.
In Private Actress, very few people know that Shiho Kobayakawa is the secret daughter of the famous actress Sayuri Nagasawa. Among them we have her nanny Makiko Mouri, her boyfriend and fellow P.A. Tomoomi, the Intrepid Reporter Matsumae, fellow aspiring actress and illegitimate child Mariko Soriarno and her Evil Counterpart Kana Juumonji/Satoka Ryoudou.
Superman's secrets have a surprisingly large number of protectors:
In the Post-Crisis era, Lois Lane discovered Clark Kent's secret identity around the time they got engaged. Afterwards, Lois was a very able Secret Keeper. In the New 52 books, however, Lois, like her Pre-Crisis counterparts, doesn't know.
Bruce Wayne is also one of Clark's Secret Keepers (as is Clark for Bruce). This led to an amusing sequence when Bruce assumed Lois knew his secret, because he didn't expect Clark to keep secrets from his wife, whereas Clark felt it wasn't his secret to share. By the end of the story Lois has become another of Bruce's Secret Keepers.
Lana Lang, in pre-Crisis what-passed-for-continuity, sometimes knew Clark's secret and sometimes didn't. In some stories she knew it but was hypnotized to only remember it in certain circumstances (such as, for example, being in the 30th century as Insect Queen... yeah, the Silver Age was like that). One 1958 story even Deconstructed why (Silver Age) Lana would make a poor Secret Keeper when she tried to cook up a new secret identity for an amnesiac Superboy as her 'cousin Andy'. Unfortunately her attempts to cover for 'Andy' backfired badly and she later ruefully concluded she just wasn't "smart enough" to hang onto the secret without blowing it. Post-Crisis onward, Lana definitely knows since Clark told her and has been a special confidant to him ever since.
Jonathan and Martha Kent started keeping Clark's secret long before he knew about it himself.
Pete Ross discovered Superman's secret ID back when he was just Superboy during a camping trip. A convenient flash of lightning revealed Clark changing clothes, and Pete kept it secret, not even letting Superboy know that he knew. He once even used his knowledge to become an honorary member of the Legion of Super-Heroes by claiming he had telepathy and knew Superboy's secret ID. Writing the identity on a lead tablet so Superboy couldn't see. Superboy assumed Pete was bluffing and that everyone else was just going along with the joke.
More people are in on Bruce Wayne's secret than one might expect:
Alfred Pennyworth is his primary Secret Keeper. He knows things about Bruce that even Dick Grayson doesn't know.
Lucius Fox is sometimes portrayed as most likely knowing that Batman and Bruce Wayne are one and the same, but not telling anyone, not even Bruce, this knowledge. In the recent movies, this was expanded to him explicitly knowing his secret and being in the loop.
In "Hush," The Riddler reveals that he deduced Batman's Secret Identity while taking a dip in a Lazarus Pit. However, Batman convinces him to keep it secret for two reasons: first, having such a secret is no fun for an obsessive puzzler like him if every one knows it, and second, it would give the owner of the Lazarus Pits, the Big Bad Ra's Al Ghul, a vital clue that the Riddler used a pit without permission and have his League of Assassins kill him no matter where he is. More recently, the Riddler has developed a mental block preventing him from remembering this.
"Hush" actually showed a lot of Batman Secret Keepers: two Robins, Superman and Lois Lane, the Huntress, Leslie Thompkins, Zatanna, Batgirl, Catwoman, Ra's and Talia Al Ghul, the Riddler...
Jim Gordon is generally portrayed as being a good enough detective to have found Batman's secret, but reserved enough not to tell anyone, even Bruce. In the first of the most recent Batman/Superman series, Batman, while noting the same thing about Superman's Perry White, reveals that he's aware that Gordon most likely knows.
In one issue of Birds of Prey, Barbara Gordon revealed her operations as Oracle to her father, who was initially stunned but ultimately proud of his daughter. Barbara then pulled out her old Batgirl costume, saying, "I'm so glad that you're handling this so well, Dad, because there's more to it, I'm afraid." Jim Gordon's response: "Okay, this part I knew."
Batwoman has her dad, Colonel Kane, who is also her main source of logistical support. Also, Renee Montoya and Nightwing deduce her identity in 52 (the former knows her quite intimately, while the latter comments that she has been leaving him clues on purpose as a sign of trust).
Mary Jane Watson was Spider-Man's Secret Keeper, then Aunt May until that entire "One More Day" thing happened.
MJ is still Spider-Man's secret keeper. They're just not romantically involved at this time.
Then again in Ultimate Spider-Man, including former bully Kenny McFarlane. Kenny actually figures it out on his own, staying up late at night, thinking about Peter's new awesomeness (ie: he flipped Kenny once, broke Flash's hand, replaced Flash on the basketball team, etc.) and Spidey's apparent availability at Midtown High School. Then he tests it. Cue the subverted Bruce Wayne Held Hostage.
Peter Parker: (to himself, while he's loading books in his locker) What the-? Oh, come on... My Spider-Sense is going off at school. That can only mean one thing. And the Academy Award for best performance by an amateur super hero trying desperately to hold onto his secret identity goes to... Kenny: [kicks him into the locker] Peter Parker: Ow! Owy ow! Why did you- ow! What is wrong with you!!?? Ow- Owww!!
Miles Morales, the new Ultimate Spider-Man, shared the secret of his powers with his friend Ganke.
Aunt May seems to play this role in the recent Spider-Man movies, particularly the second movie, where she gives Peter a speech seemingly directed at Spider-Man, which helps him decide to become Spider-Man again.
Jaime Reyes, the current Blue Beetle, was thrown a year into the future after Infinite Crisis. His family, naturally enough, freaked out on his return, so Jaime decided to prove his story about what had happened to him by revealing his secret identity. His family and friends know, but keep it a secret outside of that.
The second Blue Beetle, Ted Kord, had his girlfriend Tracy as a Secret Keeper during his Charlton incarnation.
It's revealed in DC Comics' Identity Crisis, that all the superheroes (the ones whose identities remain a secret) know each other's identities.
Whether or not the superheroes' secret keepers are aware of the others are open to debate. Especially considering that Jean Loring knows Tim Drake is Robin, despite Tim having his identity a deep secret even from Bruce's allies.
Wesley Dodds' girlfriend Dian Belmont knows he is the Sandman, and occasionally helps him in his cases.
Green Lantern Hal Jordan's first Secret Keeper was his airplane mechanic, Tom Kalmaku. When Tom accidentally learned Hal's identity, he respected GL enough to offer to let him erase the memory from his mind—but the offer convinced Hal that Tom was trustworthy, so he let him keep the knowledge and they became close friends.
In a villainous example, the Kingpin kept Daredevil's secret for years, to the point that among his goons, it was well known that he knew exactly who DD was. However, a no-name criminal decides that this is unacceptable, since Daredevil still ruins criminal operations, and thus, Kingpin is indirectly responsible for the losses other gangs have suffered. Thus, the upstart turns all of Fisk's goons against him.
In Kyon Big Damn Hero, Rika and Kyon are this to each other, Miyoko/Miyokichi inadvertently became one, and Kyon's sister wants to become one.
So far, quite a few members of the extended SOS Brigade trust Kyon's sister, but Kyon himself doesn't trust her.
He gets a lot more secret keepers in the sequel: as of the moment, Garrus, Madison, Thane, Kasumi, and Shala'Raan have all found out.
In the Elemental Chess Trilogy, it gets revealed that Maes Hughes was this for Roy Mustang and Riza Hawkeye, before and during the early part of the Fullmetal Alchemist canon. Hughes, who was Mustang's best friend, was the only one who had absolute confirmation of the actual relationship between the two (though plenty of other people guessed).
A Brief History of Equestria: Prior to the birth of Pansy's son Plow Blade, the only pony who knew about her relationship with the Earth Pony Trencher was her friend Shield Skin, who they even used as a secret messenger. This lasted for years, until Plow Blade's birth — since he was an Earth Pony like his father, Pansy was forced to admit the truth, leading to Hurricane trying to arrest and kill Trencher, which kicked off the Winter War.
Mare of Steel: The rest of the Mane Six, Princess Celestia, and Rainbow Dash's mother Firefly become this for Dash when she becomes Supermare. Thunderlane joins the list when he figures out her identity during her final fight with Brainiac.
In one of the side stories, she and Batpony become this for each other when he deduces her identity, and she uses her X-Ray Vision to see that he's the real Prince Blueblood (who was kidnapped and replaced by Zod.
The Elements of Harmony and the Savior of Worlds: Celestia and King Spykoran are the only ones aware that Spike is Spykoran's long-lost grandson. They agree to keep the secret for the time being, because they can tell he's quite happy with Twilight.
A Hairier Problem Rise Of The Furball: The Cutie Mark Crusaders initially become this for Apple Bloom when they witness her first transformation into a werewolf. Babs Seed later is introduced to the secret after accidentally witnessing it herself, and Silver Spoon has to keep it as well following her Heel-Face Turn. The other new lycanthrope, Lightning Dust, has keepers in Rainbow Dash, Applejack, Fluttershy, and Twilight, but thanks to a slip-up by Rainbow on Nightmare Night Rarity, Pinkie Pie, and Spike learn her secret as well.
In The Unknown, Cojo the midget knows Alonzo's secrets (that he is a murderer and that he is not really armless) and helps him maintain his cover in the circus.
Man of Steel: The Kents, obviously. Lois becomes one, being the only person at the Daily Planet to see Superman up close. She even jokingly welcomes him to the "Planet" when she sees him at the end.
In Now You See Me, at the end, after finding out Dylan was the fifth Horseman, Alma locks the secret up and throws away the key.
In About Time Tim reveals his secret to his sister and actually takes her back time travelling with him. She only knows briefly however; due to other circumstances Tim ends up eradicating this moment from the timeline.
In Kick-Ass Katie knows Dave is Kick Ass for most of the movie.
Parodied in Johnny Dangerously, when Johnny Kelly's mother and brother are the only people who don't know he's the notorious gangster Johnny Dangerously!
In The Wolfman (2010), Singh knows Sir John is the Wolfman and kept him restrained during nights.
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. This is most often used to conceal locations, and the house where Harry and his parents lived was guarded under the Charm so that Voldemort couldn't ever find them. They specifically chose Peter Pettigrew because everyone would naturally assume James Potter would select his brother-from-another-mother best friend, Sirius Black. Knowing that, the Potters decided to choose a less obvious Secret Keeper and Pettigrew became it. They just didn't know that Pettigrew was a Death-Eater.
Harry, himself, doesn't have a Secret Keeper in this trope's sense, which is a key reason why he's so miserable every summer with the Dursleys (who know what he is, but won't let him speak of it). Hermione's parents, being supportive of her gifts and hence exceptions to the "no parents" rule, are her summertime Secret Keepers, which spares her from feeling as isolated while she's away from school.
Zorro's mute butler effectively hides his secret identity, and helps him out on more than one occasion.
Safe-Keepers of Sharon Shinn's Safe-Keepers Series can be told any secret, no matter how terrible, no matter how much damage may be caused, and will never reveal it until the secret is found out by someone else, Or they get permission. Yes, they really will not tell. Never. Ever.
This doesn't mean that they won't do anything about it, though. Fiona in The Safe-Keeper's Secret goes out of her way to help this one girl who confides in her. However, considering that she doesn't actually have the Safe-Keeping gift, since she's Obliviously Adopted and is in fact a Heroic Bastard this may or may not count.
Deryni Stivana deCorwyn and her husband Keryell, Earl of Lendour had twins Alyce and Vera. As a hedge against persecution, they passed Vera off as the daughter of the human Howards. Alyce and Vera only learned of this after their parents were dead. They kept the secret between themselves and their tutor of ars magica Father Pascal until they Named their sons, when Alyce told her husband so he could assist in the ritual, and when King Donal paid a surpise visit to participate in Alaric Morgan's Naming. Alyce blocked Kenneth Morgan's memory at his request, Donal promised Vera that the secret was safe with him, and both Donal and Alyce died in a few months' time. Morgan and Duncan kept the secret between them for many years.
In the final books of Animorphs, the Animorphs end up revealing their powers and ongoing battle to their families as part of hiding them away from the Yeerks.
The Animorphs are also mutual Secret Keepers: their allies include a race of ancient androids who have been posing as normal humans at least since the time of the Great Pyramids, and a colony of alien freedom fighters who escaped from the Yeerks. They also know the identities of two key members of the Yeerk Peace Movement. Later, Visser One figures out that they're human (because "Andalites don't make human pop culture references!"), while they find out that she has two human children—again, they become mutual Secret Keepers for strategic reasons.
In A Song of Ice and Fire, only two people survived the battle at the Tower of Joy before the start of the book: Ned Stark and Howland Reed. There are many clues for the reader as to what Ned's big secret is, but it's clear that Howland Reed is probably the only person in-universe who knows. Knowing the author's Anyone Can DieShoot the Shaggy Dog tendencies, Howland Reed will probably still wind up dying before anybody else finds out.
In The Wheel of Time, three people know Perrin can talk to wolves: Egwene, Faile, and Elyas.(Who can also talk to wolves, and helped Perrin discover that he had the ability in the first place.) Also Moiraine knew about it too, but she was out of the picture, and believed dead for 8 books and has only just returned..
In The Stormlight Archive, Teft and Lopen end up acting as Kaladin's Secret Keepers after Kaladin first discovers his magical abilities (though Teft actually knew about them before Kaladin even did) later the rest of Bridge 4 gets let in on it, though it was kind of necessary seeing as Kaladin demonstrated said abilities fairly spectacularly. Also Shallan and Jasnah are currently mutual Secret Keepers of their shared innate soulcasting powers, and Dalinar's sons and Navani are keeping the secret of Dalinar's visions.
Several cats help keep forbidden relationships hidden. Fireheart and Mistyfoot do this for Graystripe and Silverstream's relationship, and Squirrelflight raises Leafpool's kits as her own.
In the second series, Leafpaw is the only cat that knows about both prophecies: the one that the traveling cats recieved, and the fire-and-tiger sign.
Jayfeather, Lionblaze, and Leafpool know that Hollyleaf killed Ashfur, but they decide not to reveal it to the Clan. Apparently keeping secrets just runs in the family.
One ongoing example and two plot-specific ones in the Aunt Dimity series:
Bill, Emma and Derek know about Dimity's ongoing contact with Lori via the journal (Bill and Emma have actually experienced it for themselves). To date, they have not shared this info with anyone else, not even Dimity's former attorney Willis Sr.
Gerald Willis in Aunt Dimity's Good Deed guards the remains of Sybilla Markham Willis and the journal of her brother-in-law's conversations with the dead Sybilla (after he killed her for marrying his younger brother). They were passed from father to eldest son for some three hundred years.
A major plot resolution in Aunt Dimity Digs In. Many years earlier, Italian POW Piero Sciaparelli worked for old Mr. Hodge on his farm and found an ancient Roman grave in the floor of the stables. When his daughter Francesca became engaged to young Burt Hodge, her father entrusted her with the location of it so she could protect it from vandals and archaeologists, and he gave her a replica of the bronze phalera to wear as a pendant. Later on, her younger sister Annunzia or "Annie" married Burt and took over the primary role of protector even getting her own phalera. Anxieties over which of the two might reveal the secret (the elder sister dating the archaeologist or the younger trying to save a drought-stricken farm from total ruin) drive the family animosity.
In the Rainbow Magic series, Rachel and Kirsty are this for the fairies, as is oneshot character Rebecca.
Justin's best friend, Zeke, becomes this in season 4.
The family (yep, her parents too) of Raven from That's So Raven, is fully aware of her secret Psychic Powers, most likely even before she was. These were later revealed to be a hereditary trait. The ability is treated serenely since the very first episode of the series, despite the sitcom-y mishaps that regularly occur because of them.
It's also played straight with Eddie and Chelsea.
On Torchwood, Rhys Williams (Gwen Cooper's fiance), stumbled onto proof of just what it is that Gwen does for Torchwood. Captain Jack Harkness decides to not dose Rhys with Retcon (Torchwood's amnesia pill) after he sees how much good it does Gwen to have someone outside of Torchwood to talk to about work.
In Dexter, Harry, Dexter's foster father, is the typical example. The second is the Ice Truck Killer, Dexter's nemesis, who is appropriately revealed to be Dexter's biological brother.
He gets another one in season 7 when his adopted sister Debra walks in on him killing season 6's Big Bad and he's forced to confess.
New Amsterdam: Omar (and at least one other kid of John's) in regards to their father's immortality.
On Bewitched, Darrin Stephens acts as a Secret Keeper for not only his wife, Samantha, but also for his mother-in-law, Endora, her father, Maurice, as well as Samantha's many aunts, her Uncle Arthur, and Samantha's witch-doctor, Doctor Bombay, all of whom interact with various mortals during the course of the series.
The writers of Smallville have had to let so many people in Clark's secret to keep the show fresh that Lois Lane is about the only one who DOESN'T know by now. Clark told Pete his secret in season two when the latter discovered his spaceship, Alicia revealed Clark's powers to Chloe in season four and Chloe tells him she knows in the season five premier, Lionel found out in season five, Lana started causing trouble around season six and found out in the finale, Lex found out in the season seven finale but was then Put on a Bus.
As of the episode "Isis", it's official; Clark confessed that he's the Blur to Lois, who responded by jumping into his arms, kissing him and asking, "What took you so long?"
Though it did take even longer for her to find out about the Justice League, which led to some particularly funny moments, like when Tess, despite knowing that Lois knows Clark's secret, still plays dumb because Lois doesn't know that Tess knows (and more than she does to boot). The first part of season ten gets a bit confusing at times.
Tess was a Secret Keeper as well, which was particularly important since she was oneofthebadguys, yet she still kept his secret from everyone. She had a Heel-Face Turn, though.
Pushing Daisies: Emerson and Chuck are keeping Ned's secret that he can bring the dead back to life, while Ned, Emerson and Olive are keeping Chuck's secret that she's not dead, but Olive thinks that Chuck faked her death whereas Ned and Emerson know that she did die and was brought back.
Also, Olive eventually becomes Lily's secret keeper, hiding the fact that Chuck is actually Lily's daughter, not her niece, conceived during an affair with her sister's fiance. Olive gets so overwhelmed by all the secrets that at one point she runs away to nunnery.
Gaius from Merlin is the only person (as yet) in town who knows about Merlin's powers, as well as being his mentor.
Lancelot knows too, but has agreed to keep it a secret.
Likewise, Merlin is Lancelot's Secret Keeper: he's the only one who knows that Lancelot is still in love with Guinevere.
On the same show, Gaius, Merlin and Gwen are the only ones that know Morgana has precognitive dreams.
Inverted in regards to Morgana in season 3. For the first half of the season only Merlin and Gaius know the secret that Morgana is evil, but they are unable to tell anyone about it without any proof, despite dearly wanting to warn their friends. The same thing happens with Agravaine in season 4. They both reveal themselves as traitors.
In season 5, Mordred and Merlin are this for each other, as they're both warlocks.
That's debatable. Lassiter claims to not believe and wants to expose him as a fraud, even hooking him up to a polygraph machine (which Shawn beat due to his father's extensive training). It would seem Da Chief doesn't believe it either, but Shawn is really good at solving crimes, so she doesn't care how he does it.
As of Season 5, there is also amateur profiler Declan Rand and, oddly enough, Curt Smith from Tears for Fears.
And as of Season 7, Juliet finds out, at first reacting in anger but eventually resolving to keep the secret once Shawn proves that he cares more about her than anything else, including his work at the SBPD. She also realizes that the cases he solved might be jeopardized if he told the truth about it.
Mighty Morphin' Power Rangers: the first people to discover the Rangers' secret identities were Rocky, Adam, and Aisha, and they pledged to keep them secret. They weren't able to play this role for very long however, as Jason, Zack, and Trini were soon "sent to a peace conference", and Rocky, Adam, and Aisha became their replacements as Rangers.
Although it never happened on the show, it was originally supposed to be revealed that Ernie from the juice bar had known the secret identities of the Power Rangers all along. This is why he would always give them drinks on the house as a sign of appreciation.
Power Rangers Zeo: Tommy, while morphed, tries telling his brother David that his unmorphed identity is safe, but doesn't believe him until Tommy demorphs in front of him. Presumably he keeps the identity a secret, though it is possible that David's father is also aware of Tommy's identity as a ranger.
In "Three's a Crowd", Kendall, a girl Black Ranger Danny is madly in love with, as well as a rich young man trying to win her love, both discover Danny's identity. Neither of them show any desire to reveal his secret, but the would-be lover does tell Danny that he shouldn't date Kendall due to his responsibilities, and while it's somewhat implied that he's just being a Jerk Ass so that he can have Kendall to himself, Kendall ultimately says the same thing at the episode's end.
The following episode, "A Father's Footsteps", has White Ranger Alyssa's father, a martial arts master whose specialty was a family tradition, come to visit. Alyssa runs very late for their meeting, and he leaves, only to see the Power Rangers fighting the Monster of the Week, and recognizing Alyssa by her martial arts moves. They share a heartfelt moment in the end, but Mr. Enrilé only praises her by calling her his "white tiger"; as the family's martial art was referred to as "Tiger Style", it's unknown whether or not Alyssa finds out he knows.
Power Rangers Ninja Storm has Red Ranger Shane's older brother, a very successful businessman who is somewhat distant from Shane, discover his secret. He had previously complained about Shane not having responsibility, but revises his whole attitude upon learning the secret. Unfortunately, he's a one-shot character.
Power Rangers Mystic Force has Toby, the rangers' boss at their civilian jobs, but he doesn't find out until the fourth-to-last episode…though they did use a Sarcastic Confession on him at an earlier point to explain why they continuously ran off during their shifts. Subverted, however, in that he pretty much tells the whole town in order to gather enough energy to give the Rangers the final boost in power that they need to destroy the Big Bad.
The Scoobies are Buffy's Secret Keepers, but they all stretch the definition of "normal". Many of the Scoobies start outside the fold as "normals" and eventually get pulled in to Scoobiedom (Cordelia, Oz, Tara). Another example would be Buffy's mother Joyce after Season 2.
Principle Snyder knew about the existence of the Hellmouth all along - and was even charged to keep it a secret from as many people as possible by the Mayor himself. That said, he was completely oblivious to Buffy's identity or the Scooby Gang.
In Wolfblood, Maddy and Rhydian act as secret keepers for each other. At the end of season 1 Tom and Shannon find out, but agree to keep it a secret
Hannah Montana's secret keepers (aside from Bobby Ray and Jackson) are Lilly and Oliver. Later, there's Jake Ryan, Lilly's mother, Oliver's mother, Officer Daria and his daughter. That is until everyone in her hometown (plus one British journalist and his daughters) discovers the fact in the movie... and by the end of the series, everyone in the world knows it.
Almost all the major Muggle characters on Kyle XY end up being this, although in this case it's also to help Kyle and Jessi. At one point, it causes a Crowning Moment of Funny for Josh as he spills all his other secrets. Josh also briefly doubles as one for Andy, before her cancer is made public.
In Fringe, Agent Lincoln Lee from the Alternate Universe Fringe Division prides himself as this when he is speaking to (Alternate)Olivia. Later, he decides that he "can't keep secrets for shit" and spills it all to (Alternate)Charlie Francis.
Aquila: Mrs. Murray discovers the ship and keeps the secret in the second season.
Warehouse 13: It's far from unheard of for an outsider to discover the Warehouse in some way, but it hasn't been made into a major plot point yet; as of yet, most of the outsiders that have learned enough to be a threat have either been recruited or killed; the only known exceptions as of yet are Claudia's brother, Mrs. Frederic's grandson, and Monty the Magnificent, an elderly magician that was let in on the secret due to his passion for "real magic." However, the series also has a Chekhov's Gun that has yet to be fired: each Warehouse agent is allowed to reveal the true nature of their work to exactly one living person. But as was said earlier, it hasn't been made into a major plot point yet.
In The Listener, Mercer, Oz, Marks and later Olivia is this for the mind-reading Toby. In season two, Detective McCluskey and Dev Clark join the team.
Jonathan is this for the ghostly Ainsworth sisters in Dead Gorgeous.
In the TV series adaptation of The Green Hornet, Lenore "Casey" Case and District Attorney Frank Scanlon keep the Hornet's and Kato's secret identities (Scanlon parallels the use of Police Commissioner Higgins as a secret keeper and law enforcement contact in the original radio series).
Late in the radio show's run, Lenore Case learned about Reid's second career as the Hornet and helped cover for him.
In Free Spirit, Gene and his older siblings, Robb and Jessie, become forbidden from telling anyone about their nanny, Winnie Goodwin, performing witchcraft.
Charmed: There's a fairly extensive list of mortals who know that magic is real, and specifically that the Halliwell sisters are actually witches. The most notable examples are Victor Bennett (father to three of the four sisters), Inspector Andy Trudeau (oldest sister Prue's Love Interest)note Though he only acts this part for about half of season one, at the end of which he dies, Inspector Darryl Morris (Andy's partner), Homeland Security Agent Kyle Brody (another Love Interest, who eventually dropped from this trope when he died and became a Whitelighter), Homeland Security Agent Murphy (who figured out that something was up when the sisters faked their deaths and subsequently helped them "come back from the dead"), and Henry Mitchell (youngest sister Paige's boyfriend, who she eventually married). In the season nine comics, Phoebe's boss Elise Rothman also becomes a Secret Keeper when the sisters save her life (Paige had to heal her, and then they had to magically make her look like she was still injured in order to properly prosecute the man who had attacked her).
Mrs. Roper on Three's Company finds out early on that Jack isn't actually gay, but keeps the knowledge from her husband so Jack can continue sharing an apartment with two women.
In the Arrow universe, Barry Allen and Oliver Queen are mutual secret keepers for each other. Both heroes also have a small team of allies who know their true identities.
A bit of Irony considering their later relationship in Gotham where Bruce Wayne and Alfred are so far the only two who Gordon revealed that he intended to bring down the corruption of the police and the city and that he was planning to play along inside the system to do it.
Dino Attack RPG has the siblings Bart and Carrie Enderson, both bartenders who both kept secrets for a different mercenary. To add a cruel twist of irony, neither knew about the other's activities (at least not until after the war and both mercenaries were dead), and one of their clients was determined to kill the other.
In the second game of the Eagle Eye Mysteries PC Edutainment Game series, protagonists Jake and Jennifer Eagle, as well as your unseen player character, become Secret Keepers in "Case of Blood's Bold Bauble." A distant relative tells you that there's a particular story that has been kept within his immediate family for generations: that three centuries ago, an ancestor found a small but very valuable ruby that was dropped by the titular thief Captain Blood and his gang, who beat a hasty retreat after attempting to break into the Tower of London to steal the Crown Jewels. The ancestor, fearing he would be implicated in the robbery if he tried to return the ruby, kept it for himself and only told its story to his son, who in turn told the tale to his son, and so on. By the time you, Jake and Jennifer have been told the story, the secret's origins are so distant that the ruby's revelation can no longer hurt the family; the only reason the relative tells you the story is because he thinks one of his three children, who are currently the only other people who know the family secret, might have broken the bond of parent-child trust and told the secret to cash in on it—hence, the beginning of your covert investigation to find out who's responsible without tipping off any of the children as to your knowledge. Turns out none of the kids talked—the oldest son is set to star in a film about a very different Captain Blood, and the misunderstanding arose from that.
In Red vs. Blue: Reconstruction, Agent Washington hides the fact that he has all the memories of the Epsilon AI, which is the manifestation of the Alpha AI's memories, which itself was created from the Director's mind. This means that he has knowledge of both the torture used to split the Alpha into separate AIs and all of the access codes used in the Director's lab.
Gologotha is the only one who knows how Gaea got her avatar back after getting banned permanently from the game for borderline illegal activity until an unexpected third party makes the bargainning chip's existence public.
Judge Dead is all but stated to have known of the secret revealed in the Wham Episode all along.
Justin is not only the Secret Keeper for the core group, keeping their activities secret from everyone (especially George), he's also keeping Noah's Secret Identity as the Child Left Behind a secret even from his closest friends.
In The Dreamer, Yvette is the only person who knows about Beatrice's time-travel dreams.
In Electric Wonderland, Aerynn Arlia's friends and co-workers must make sure that the authorities of Cyberspace never find out about her performing illegal witchcraft.
Amanda Waller: That's a national security matter. And if I were you I wouldn't probe the situation too closely... rich boy.
Ah-Mah and Ray Ray Lee Juniper Lee's grandma and little brother respectively, and later her older brother, Dennis Lee. (Although they're not quite regular Joes — both can see magical creatures, and Ah-Mah was June's predecessor. One episode reveals Ray Ray's ability as being an accident. For Dennis, he can only see magical creatures when he wears a talisman.)
In Hey Arnold!!, Phoebe, Patty, Trish Wittenberg, Grandpa Phil and Brainy all know Helga Pataki's most dark and deep secret: her overwhelming love for Arnold.
As well as Helga's shrink Dr. Bliss, who is a Secret Keeper by way of professional confidentiality.
Helga admits it to Lila as well, threatening her with violence so she wouldn't tell.
Subverted by Gaz on Invader Zim. She discovers early on that Zim is an alien, but doesn't care. When Dib says "Zim's trying to conquer the world," Gaz replies, "but he's so bad at it." Which is true, but...
Trixie and Spud, from American Dragon Jake Long. In the episode where they find out about Jake's dragon powers, Jake's grandfather gives him a potion that will erase their memories and put everything back the way it was to maintain the Masquerade around the magical world. In a subversion of Status Quo Is God (and possibly as a "screw you" to Juniper Lee, which had a reset after her friends found out), he pours it out in a potted plant rather than dose their drinks, and they continue to help him in both forms.
The Holograms (Kimber Benton, Shana Elmsford, Aja Leith, and Carmen 'Raya' Alsono) from Jem and the Holograms count here as well.
He-Man also, at the Sorceress' request, keeps the secret that the Sorceress is Teela's mother.
Both averted and played straight in Kim Possible. Averted in that Kim's hero status is publicly known. Played straight with Ron Stoppable when he attended a ninja school; he promised Yori and Master Sensei that he wouldn't tell anyone about the school, even Kim. Later in the series, Yori allowed Ron to tell Kim (who had already discovered the truth about the school on her own). Presumably Kim then became a Secret Keeper for the school as well.
In Gargoyles, Eliza Maza and, later, Matt Bluestone work to conceal the existence of Goliath and his clan from humanity, as does David Xanatos when he's not trying to kill them. By the end of the second season, they fail spectacularly.
Freakazoid! had Roddy and Cosgrove, and later Steff and Professor Jones as of "Mission: Freakazoid" when Cosgrove let it slip.
Phineas and Ferb has Stacy Hirano becoming this for Perry in "Happy Birthday Isabella" to avoid having her memories erased, and having to sit through a scary movie she watched again.
Lawyers (at least in the United States, the United Kingdom, and some other common law jurisdictions). While there are a few exceptions to attorney-client privilege (generally having to do with preventing reasonably certain serious bodily harm), for the most part, an attorney can't reveal anything that their client tells them in confidence. This means that in most, if not all, U.S. jurisdictions that if an attorney find out their client committed a crime that an innocent person's in jail for, their ethical obligations to their client prevent them from saying anything without their client's permission, even if the other person is serving a life sentence. Keeping Secrets Sucks.
In response to this dilemma some state bars have added rules that allow for limited disclosures in this situation, arguing that justice demands the innocent go free.
Similarly, although doctors are required to report evidence of a crime (gunshot wounds, suspected child abuse, etc.), most details of your medical history are protected by doctor-patient privilege.
Catholic priests are bound by The Seal of the Confessional to never divulge what anyone says in confession, even if they confess to a crime. A priest violating the seal would risk laicization (defrocking) and excommunication (which means that he could not receive any sacrament until he would (ironically) confess his violation of the seal).
There have been cases in the Papal States wherein the authorities broke the seal of the confessional, at papal direction, to arrest and prosecute murderers.
In the United States, the Supreme Court has ruled that gathering information by violating the Seal is a violation of the First Amendment and as such unadmissible in any court.