A person who, for a multitude of possible reasons, is hidden from the rest of society.
Perhaps they're born into a world with a strict limit on family size, and their birth exceeded that limit. Maybe the government considers their very existence dangerous, or maybe they're trying to protect this person from everybody else. Maybe this person is just a victim, being ostracized from everyone, or they are willingly kept hidden through their own free will. They may be born into it, or forced into the situation later in life.
Whatever the reason, people don't know this character exists, and other characters want to keep it that way. Sometimes the character is the protagonist, and the story will either detail their escape from exile or alternatively their attempts to stay safe and hidden. If it's not the protagonist, it'll be someone they meet on their journey, often who they need to set free. In some cases, this character may actually be the villain, kept hidden to keep others safe.
Often the result of someone being un-personed, where someone is removed from public, government, and/or historical records (occasionally by their own hand), especially in long-term cases. Contrast Secret Identity, where the person has one or more public identities and the secret is that they're all one person, and The Spook, an Un-person who still moves about in the larger world in secret, as opposed to being kept secluded. Anyone imprisoned (or even just stationed or employed) in a top-secret Black Site will tend to become this after enough time has passed. The Masquerade may hide members in this way, to protect their secrets.
This can have a few results for the character in hiding, depending on the circumstances. Some may develop a "public" identity for the purposes of interacting with the outside world, while others may go through Sanity Slippage or desperately try to find a way to meet other people regardless. Others, especially those doing this willingly, will be content.
Supertrope to Batman in My Basement, Madwoman in the Attic, Witness Protection, and Hidden Backup Prince. Compare The Hermit. Don't confuse with Anti-Interference Lock Up, where a character is merely restrained in some way, not completely hidden.
As this trope often deals with a Reveal, there will be Unmarked Spoilers Ahead.
- One Piece:
- Portgas D. Ace's mother Rouge held him for twenty months in her womb just to make sure the public won't know he's the son of her lover, Pirate King Gold Roger. Because of the stigma that a pirate's son will Turn Out Like His Father let alone him being the son of Roger and that the Marines, after Roger's death, started a witch-hunt of every woman ever affiliated with him, searching for the possibility of an offspring. This costs her her life.
- As a favor of Roger himself to his rival, Vice Admiral Garp, the latter also tried to keep Ace hidden and was there when Rouge gave birth to him. Then he brought Ace to his hometown in East Blue and got Curly Dadan to raise him (along with the young Luffy) with the intention of making them Marine officers. Ace himself also tends to go by his mother's maiden surname rather than his dad's.
- As far as those who live in the territories of the World Government know, the Five Elders are the highest authority in the world. But the Reverie Arc revealed the existence of someone who the Five Elders answer to, Imu. Imu's existence is kept secret to maintain the lie that there is no singular ruler of the world.
- Yasu from Umineko: When They Cry is kept secret by different groups for different reasons. Natsuhi and Kinzo keep Yasu secret because Yasu is Kinzo's bastard child, and Natsuhi threw Yasu off a cliff. Genji and Kumasawa, on the other hand, keep Yasu's existence secret partially at Kinzo's wishes, but they keep secret even from him and Natsuhi that Yasu survived the cliff fall because they think that Kinzo might eventually fall in love with Yasu, as he did Yasu's mother, Beatrice Ushiromiya.
- In The Hunchback of Notre Dame, Judge Frollo actively tries to keep Quasimodo hidden from the rest of the public and makes him stay in the tower by convincing him that the world is a cruel place and he is safer in the bell tower. Of course, this only works up until the Feast of Fools, and stories are still told about Quasimodo, making him legendary despite the fact only Frollo and the Archdeacon really knew he existed.
- At the end of Edward Scissorhands, Edward is sent back to his castle at the top of the hill and Kim tells the townspeople, who want him executed after he's framed for a crime, that he died.
- In the Slasher Movie Humongous, a woman named Ida was raped and kept the child born of the rape hidden away...until she dies.
- None Shall Escape: When Wilhelm comes to Lidzbark as Reichskommissar, he starts looking for Jan (who has been wounded in battle during the Nazi invasion of Poland)—no doubt to exact revenge for the stone throw that cost him his left eye twenty years previously. When she finds out about this, Marja sends Janina to tell Jan to go into hiding. Jan eventually hides out in Marja's basement.
- At the start of A New Hope, Luke Skywalker has grown up at his uncle's desert farm on Tatooine, hidden from The Empire until he can be trained as a Jedi. His sister Leia, on the other hand, has been hiding in plain sight as the adopted daughter of Alderaan's queen and prince consort.
- The Man in the Iron Mask in The Vicomte de Bragelonne is a twin brother of King Louis XIV, who has been kept locked away all his life so he couldn't usurp his brother's throne. Of course, he is freed and does just that in the course of the novel.
- In A Brother's Price, Jerin's grandfather Alannon was kept secret by the grandmothers ... he was a prince, although not a Hidden Backup Prince (he was not heir to the throne), and their kidnapping him from a palace under siege was not entirely legal. They were determined to keep him, even though the royal family (the one that had wiped out his branch of the family) searched for him for a long time.
- In the Declaration Trilogy by Gemma Malley, Peter has been hidden by his parents because of the Childless Dystopia they live in.
- This is what happens to the younger of the Princes in the Tower in the I, Richard Plantagenet Series. The Duke of Buckingham arranges for the princes murder, but the younger escapes and is sent abroad under the name Perkin.
- Shadow Children: Due to a strict policy where parents cannot have more than two children, any third children born must be kept secret from everyone; main character Luke spent the entire first book just hiding out in his family's home, and met a third-child girl across the street who did the same.
- In The Belgariad, an assassination attempt on the Rivan royal family nearly killed the entire family. The lone survivor, the young Prince Geran, was rescued by Belgarath and presumed dead by the world at large. He and his descendants went into hiding as commoners to ensure Garion could one day be born to fulfill the Prophecy and reclaim the throne.
- At the beginning of Mermaid (2011), Princess Margrethe and her lady-in-waiting Edele are both living in disguise as novice nuns to protect them in case of a war with the Southern Kingdom. Both are bored and lonely, even though they're given more freedom than the actual nuns.
- ‘’Johanna Mason: They Will Never See Me Cry’’  Croft, the eldest victor of district 7 hides the birth of his daughter and grandchildren from the Capitol to protect them from the reaping, hiding them inside his house for their whole lives and only letting a select few know they exist.
- ‘’The Victors Project’’: Happens now and then with Rebels. After Wonder is Driven to Suicide Luxe and Platinum snuggle his sister out of the district after President Snow and Luster Lancaster kill the rest of the family to deter any other victors from following Wonders example. It’s also mentioned that Beetee sneaks in runaways from some of the rural districts and gives them new identities in Three.
- Because the Ark on The 100 enforced a strict one-child policy, with violations resulting in execution, Bellamy and his mother kept his illegal little sister Octavia hidden under the floor the entire time she was growing up. When she was found out, she was thrown into lockup, her brother was demoted to a janitor, and their mother was Thrown Out the Airlock.
- In later seasons we also meet Madi, Jordan, and Hope, also all hidden as children for one plot reason or another. Hope being hidden by Octavia, the original hidden child of the series who is now an adult, also makes for a nice full-circle moment.
- In the Decoy episode "Scape Goat", a woman embezzles money to hide her mentally disabled five-year-old son in a private institution. She doesn't tell anyone about him for fear of being blamed for his condition.
- Doctor Who: The Doctor meets a woman named River Song, who keeps the nature of his relationship with her secret because it hasn't happened yet for him. As the series progresses, he learns more and more about her, culminating in the pivotal reveal that she was also a secret from her parents, and the Doctor was instrumental in rescuing her from her kidnappers. Her real name is Melody Pond, the daughter of Rory and Amy Pond, the first companions he recruited after he met River for the first time.
- Ezel: Gambling kingpin Ramiz Karaeski kept the existence of his daughter Azad a secret, sending her into hiding as a child to prevent his enemies from using her life as leverage. Considering her three older brothers were assassinated by his greatest rival, his choice is not unwarranted.
- Supernatural: In Season Five, The Winchesters and Castiel meet a young boy name Jesse who is a demon/human hybrid with incredible power, who is essentially the Antichrist. Castiel wants to kill him but he's a decent kid who doesn't understand his power. So, instead, they send him into hiding in Australia.
- WWE: When the Bella Twins debuted, it was only Brie Bella who officially showed up. During matches when she was losing, she'd crawl under the ring and her sister Nikki would come out to finish the match.
- Dungeons & Dragons, Forgotten Realms setting. When Prince Haedrak Rhindaun of Tethyr was born, his existence was kept secret. When he was only six months old, his name was changed to "Lhaeo" and he was sent to the mage Elminster for safekeeping. Twelve years later, the populace of Tethyr rose in rebellion and killed the entire rest of the royal family, leaving Prince Rhindaun as its last member. He stayed in Elminster's care until he was 33 years old, when he was convinced to begin his attempt to regain the crown of Tethyr.
- Chzo Mythos: Roderick DeFoe left his mentally handicapped son John in a secret cellar behind the kitchen, often beating and neglecting him out of hatred for him "killing" his mother during birth. Not even his other son and John's twin brother, Matthew, knew about this for much of his life. When Matthew did find out, Roderick tried to convince him that there never was a boy behind the kitchen.
- Fire Emblem: Path of Radiance: Elincia's birth was kept from the public eye of Crimea, because she was born when the King appointed her uncle Renning as the next heir, and it would cause problems for the country. Only the leaders of other countries know she exists.
- In Fate/Grand Order, Minamono-no-Yorimitsu was kept hidden for years due to her supernatural heritage as the mortal-born daughter of Kitano-Tenjin and the Gozu Tenno, giving her oni blood. But her immense talents and her mortal father's lack of an heir led him to take her back and reveal her to the world as his "son".
- Jade Empire: The Big Bad secreted away and kept the last of the Spirit Monks hidden from the world and his siblings while he concocted a master plan that would involve using the said monk as means to achieve the title of Emperor, and ultimately, godhood and immortal life.
- In Crusader Kings II, characters can go (or be sent) into hiding if there is reasonable evidence for a plot on their lives. This option is available to both the player and the AI; while active, assassination plots cannot trigger. However, going into hiding is not without its drawbacks. Characters will take prestige hits while hidden, and those who are in hiding for an extended period of time may become stressed, depressed, or even go insane.
- In AdventureQuest Worlds, Sepulchure, the Doom Knight and overlord of evil, kept his daughter Gravelyn secret for years, with her existence only becoming common knowledge after his death and her ascension to the throne of the Shadowscythe Empire. The reason for all this secrecy isn't known until the second part of the Doomwood saga where it's revealed that she is the Champion of Light, the only thing keeping the Realm of Darkness from destroying the world, and the reason that Sepulchure didn't kill her was out of love for both her and her mother Lynaria.
- BlazBlue: Chronophantasma: A subplot for Bang has him trying to find his old master's child, the true heir to the throne of NOL's leader. Later in the story, it's revealed that said child, Homura, has been in the custody of Bang's friend, Kagura, ever since Homura's parent was slain in the Ikaruga Civil War, and has been hidden from the public since.
- In a Treehouse of Horror episode of The Simpsons Homer & Marge keep Bart's Evil Twin Hugo hidden and locked up in the attic.
- In Avatar: The Last Airbender, Toph was this until meeting Aang. Her overprotective parents kept her locked inside the grounds of their home (although she was able to sneak out to work as a wrestler).
- The Thomas & Friends special Blue Mountain Mystery revolves around the Narrow Gauge engines keeping a lost engine, Luke, secret from the Thin Controller (due to supposedly killing another engine via an accident said engine was found and repaired as Victor, who resolved everything when he found out). How this is managed isn't quite elaborated upon, since the engines (including Luke himself) are pretty much completely reliant on workmen to function.