Wendell the Baker for Cinnamon Toast Crunch used to have two other bakers named Bob and Quello/Quienno by some, but they disappeared for reasons unknown around the early '90s, and were never featured again.
Initially, the premise was that Wendell has wandered off exploring the world, and the other two were searching for him (there was even a "Where's Wendell" contest). But the campaign became a sort of Aborted Arc without a resolution, and eventually the "temporary" ads featuring only Wendell just became the new normal.
McDonaldland and the vast cast of characters who once existed side-by-side with Ronald McDonald in McDonald's commercials were steadily eliminated from the 1970s onward, until only the "core cast" of Ronald, Grimace, Birdie, and Hamburglar remained. For awhile past 2000 now, McDonald's ads have only featured Ronald, and lately even he hasn't been seen terribly often.
The Burger King Kingdom was BK's answer to McDonaldland, and featured a colorful cast of characters including the milkshake-craving knight Sir Shakes-a-Lot, the picture-framed Burger Thing, the robotic Wizard of Fries, the skeptic Duke of Doubt, and The Burger King himself. They were phased out in favor of the Totally Radical BK Kids' Club in the late '80s, which itself disappeared shortly into the new millennium.
Cookie Crisp cereal had in its early marketing campaigns a bobby who chases after a bandit-masked thief and his bandit-masked dog. Eventually the thief vanished, followed promptly by the bobby, leaving just the dog (still wearing his bandit mask, oddly enough). Now the dog has completely vanished, having been replaced by a wolf named Chip (ironically the same name as the dog that preceded him).
Erin Esurance was a Cartoon Network-esque super spy who went on dangerous assignments while pitching for the insurance company that created her. Her commercials ran for several months then abruptly stopped without explanation, and she was reduced to a static icon next to the company's name before being dropped completely. Much has been written about the vast, vast quantity of Internet porn she's now in, but the simple truth is most likely that the campaign had run its course and the company couldn't think of any new adventures for her.
Ned The Incompetent Loan Officer was the face of Ditech in the early-2000's. The commercials would usually show him with a clientnote in a few commercials, the client would be his mother - strangely played by the exact same actor that played him, attempting to give him/her a home equity loan. He would horribly botch the deal, and the client(s) would call Ditech instead for the loan, leaving him to moan "Lost another loan to Ditech." Around 2003, the character inexplicably disappeared from Ditech's commercials, most likely because the company ran out of ways to use him.