The opening sentence of the novelization of The Last Jedi may have caught readers off guard the first time they opened the book. "Luke Skywalker stood in the cooling sands of Tatooine, his wife by his side." The sentence starts a scene quickly revealed to be a dream, depicting a version of Star Wars where Luke never left Tatooine. One where Leia was never rescued, the Death Star was never destroyed, and the Empire successfully destroyed the Rebel Alliance. It's an interesting scene that points out how different everything would have been, including a mention that Jabba was still alive and controlling vast portions of the Tatooine economy. But that mention of Luke having a wife really sets the whole scene off-kilter.
The character mentioned as Luke's wife, whose name is Camie, wasn't just made up for this alternate timeline dream sequence that starts off the book. In fact, the character has been a part of Star Wars from the very beginning, all the way back in 1977. But there's a very good chance that you've never seen her.
Camie first appeared in a scene in A New Hope that was eventually cut from the movie. In an earlier version of the film, Luke is introduced much earlier and witnesses the spaceship battle from the surface of Tatooine. He quickly goes to tell his friends, a group that includes Camie as well as Biggs. By the time they go outside to look, the battle is over, and nobody believes him. It's a scene that would have introduced Biggs much earlier and impacted his return at the end of the movie to fight (and die) in the Battle of Yavin. Ultimately, the scene slowed the first act's pacing down, and the film was better with it missing.
Still, just because the scene was cut from the movie doesn't mean it didn't happen. Due to the success of Star Wars, the story of A New Hope has been told several times in different formats. And in many cases, that scene is still a part of the story. It's in the novelization of the movie, as well as the Marvel comic adaptation. It's also in the radio adaptation that aired on NPR. In both cases, Camie's character was present.
Over the years, there have been several retellings of the events of A New Hope, either in comics, picture books, or young reader novels designed to be journals from different character perspectives. In several of these cases, the scene where Luke talks with his friends and Camie is present. Because of the continued character use, she began appearing in other pieces of Star Wars Legends. She appeared in the video game Star Wars: Galaxies and was referenced in the novel Darksaber.
In 1989, a short story told from Camie's perspective was published in Galaxy Guide 1: A New Hope. The book was published to supplement the role-playing game and featured several stories from various characters telling their perspective on events to an in-universe galaxy historian.
Eventually, Camie would return to the screen years later in an episode of The Book of Boba Fett. In the second episode of the series, a group of Nikto bikers are harassing two people at Tosche Station when Boba Fett intervenes and takes care of the bikers. Those two bystanders are Laze Loneozer, another friend from Luke's childhood who appeared in the deleted scene, and Camie.
In the new Disney canon, Camie was given the last name of Marstrap, which used to belong to another of Luke's friends named Windy. The official last name of Windy has been confusing through the years with some sources saying "Marstrap" and some saying "Starkiller." Confusing things further, some sources have given Camie the last name of "Loneozer," though that might be because it's implied that Laze and Camie were married in the years after Luke left Tatooine.
In the end, Camie is one of several characters who would have never seen the light of day in any other scenario. A group of friends that were only in one scene, and that scene was deleted before the movie was ever released in theaters, should result in the characters all being forgotten about. But in Star Wars, it seems like every character gets a chance at a fully fleshed-out backstory, meaning that Camie has taken on a life of her own.