Chuck Cunningham Syndrome
Generally, if writers want to remove a character from their ensemble, they will either kill that character off or put him on a bus (or both) to explain their absence. Sufferers of Chuck Cunningham Syndrome, on the other hand, simply disappear into limbo. They will often be retconned right out of the story’s history, and everyone still left on-screen will simply carry on as if the character never existed or had any impact on the story. This is sometimes caused by the writers gradually losing interest in the character and, without making a conscious decision to remove them, eventually forget about them entirely. More often, complications behind the scenes drive the decision to remove a character.
The Trope Namer is Chuck, who was actually Richie Cunningham’s older brother for two seasons on Happy Days. Remember him? No?
In recent years, though, as media has become more meta, playful references to the ignominiously departed have become common, either as lampshades within the series itself or in parodies or satires of it.
A subtrope of Un-person, although this trope isn’t necessarily intentional. Similar in spirit to The Other Darrin, wherein a character’s actor or voice is replaced and nobody in-universe seems to notice or care. Also see Out of Focus, when a character is gone but not quite forgotten; Sequel Non-Entity which is a subtrope that’s concerned with a character not returning for a sequel in particular rather than vanishing from the series as a whole, and Shoo Out the New Guy, who gets at least an excuse in the show for disappearing. Contrast with Remember the New Guy, where someone comes in out of nowhere sometime midway through and the series acts as though they’ve been there the whole time. For characters who are written out of the main story but are still hanging around in view, see Demoted to Extra. For characters who are specifically brought in for a one-shot purpose, see Long-Lost Uncle Aesop. For characters that are given a reason for their departure and an on-screen send-off, see Put on a Bus. Compare Forgotten Fallen Friend. If a character appears in the pilot or very early episode of a show and then vanishes it may be a case of Early Installment Weirdness. See also Absentee Actor. Present Absence is when this is averted and a character’s absence is not only noticed in-story, but is often a major driver of the plot. What Happened to the Mouse? is this for minor aspects like side characters or subplots.
This trope is only applicable to genuine members of the ensemble who would otherwise seem to be a little more permanent, not characters who appear out of nowhere for one or two episodes and vanish just as swiftly.
When this trope happens in-universe, with the character having his existence literally erased, it’s a Retgone.