Snuffleupagus from Sesame Street. Now everyone can see him and not just Big Bird, so children won’t have to fear that adults will think they’re lying when they tell them their “secrets”.
They showed him in the very first episode-hell, they even told him who #1 was.
Chuck was the opposite of the thread’s premise, actually. He appeared in the first few episodes then disappeared.
Or in the case of Dr Hoffman, they originally say “Julius Hoffman” is coming. But then series creator Dan Curtis met Grayson Hall, the actress wife of writer Sam Hall and said “You have a great face, I must cast you” as Julia Hoffman. Nobody will remember that it was originally Julius.
On “Stargate SG-1” the Furlings are mentioned but never really shown, except for the fictional; “Wormhole X-treme”. But who makes a movie of a TV show cancelled after three episodes?
The “Maverick” TV show had Brett and Bart quoting Pappy , who didn’t show up until episode 54 (played by James Garner).
Elizabeth Bartlet, eldest daughter of President Bartlet on “The West Wing” is mentioned a number of times, beginning with the first season.
Her character does not appear until the first episode of Season 5.