Admit it: Did you watch Will & Grace for Will and Grace? Or was it Jack & Karen that really carried the series?
Ramona Quimby was introduced in Beverly Clearly’s Henry Higgins series as a little sister to Henry’s friend Beezus. As the author wrote
Hannibal Lecter was a minor character in Thomas Harris’s Red Dragon, and the protagonist is his next three books.
Urkel.
For me, Joey and Phoebe stole “Friends” fairly early on, and ran like hell with it!
^ Will & Grace: Karen, Karen, Karen, Karen, Karen…
“Just Shoot Me”: Finch walked off with it (not hard to do).
Well…Karen.
Not entirely true; Lecter is at best a supporting character in Silence of the Lambs. That story is about Starling, and she gets pretty much equal time with him in Hannibal. I shan’t comment on Hannibal Rising, as I was far too vexed by Hannibal-no-adjective to read the former.
I’m not sure I’d call them supporting characters, though. Friends was a show with six leads.
Answering the OP’s question, the first show that occurs to me is Happy Days, and Fonzie.
The Fonz.
Det. Rizzoli was a very minor character in the first book about ME Maura Isles. I’m not sure if that’s what you’re going for, though.
Michael J Fox in Family Ties. This was supposed to be Meredith Baxter-Birney’s show.
Alex P. Keaton quickly stole the lead.
Normally, I would agree with you, but it seemed they were trying to push Aniston into the “lead” slot, and the Joey and Phoebe characters were secondary to the others. I dunno, I need a nap.
Popeye was a minor character when his strip first appeared.
Loki, in Thor/Avengers/Thor II.
Captain Jack Sparrow; to me, it read as if the girl was supposed to be the main character, but it didn’t turn out that way…
Spock was a much bigger hit than they ever expected him to be.
Willow kind of outdistanced Buffy once she got her magic up to addiction levels.
Richard Pryor’s character in Silver Streak.
It was fairly common in newspaper comic strips.
Snuffy Smith was a minor character in the Barnie Google strip. Now that particular Google appears in the strip only every couple of years, if that often.
Big Chief Wahoo was a newspaper comic strip that was part humor, part mystery. Wahoo met up with Steve Roper in one episode Slowly, Wahoo was reduced to being a sidekick and the title of the strip was changed to Steve Roper; Wahoo was shown less and less. But Roper had the same thing happen to him: another, younger adventurer, Mike Nomad, took over the lead and the strip retitled to Steve Roper and Mike Nomad. Despite second billing, it was Nomad’s strip.
The most famous in comics, though, was George Herriman’s The Dingbat Family/The Family Upstairs, Herriman started doodling an additional strip at the bottom showing a mouse throwing bricks at a cat. These quickly spun off to be Krazy Kat, often considered the acme of comic strip art.
That reminds me-- I remember reading that Opus was just supposed to be a one-off character to show what a weird kid Binkley was because he had a penguin for a pet. By the time it ended, Bloom County might as well have been called “Opus and Friends.”
Lord John seems to be taking over the Outlander books. I guess Jamie and Claire are getting… well… long in the tooth.
Steed was the partner of the main character in the original “The Avengers,” who eventually took over the series.
When I was a kid, I thought that the title of the Peanuts strip was “Snoopy”, because he was obviously the main character.
Snoopy, Spock, Fonzie, Alex Keaton and Urkel are all good examples of this.
Sheldon Cooper in The Big Bang Theory.
Although Fonzie is pretty much the prototypical answer, Barney Fife is an earlier example. Originally, the show was much more about Andy, but the show’s producers (including Andy Griffith) saw how great Don Knotts was, and also saw the public reaction, and significantly increased Barney’s role. Barney never took over the show to the extent Fonzie did in Happy Days, but it is an earlier example. Somewhat ironic that a later Barney took over How I Met Your Mother in a similar fashion, due largely to the talent of the actor.
He was always one of the three main characters, and he hasn’t overshadowed Penny and Leonard - Howard, Raj, and the ladies other than Penny are closer to being examples, though they’re not very good examples, either, since they still sit behind the main three.
Seven of Nine in Voyager.
Elmo on Sesame Street.
Miss Piggy on The Muppet Show. She was supposed to be a one-shot, who slapped Kermit. Instead, Frank Oz karate-chopped him, and a star was born.