Many of the characters from The Simpsons, including Krusty The Clown, Mr. Burns and Smithers, Apu, Moe, and many more are considered secondary characters, but they have all reached high popularity, including full episodes revolving around them. There are even a few characters, like Groundskeeper Willie and Cletus The Slack-Jawed Yokel, who were originally intended as one-shot characters, but were popular enough to become recurring characters and later have their own episodes.
Another one I can think of is Foghorn Leghorn. He first appeared in the 1946 cartoon Walky Talky Hawky, and was intended to be a secondary character in contrast to Henery Hawk, who was intended to be the star. He reached star power with the audience however, and now he is one of the most well-known Looney Tunes characters, whereas most people don't even know Henery Hawk's name.