Another sitcom which has an 'invisible character' is 'Keeping Up Appearances.' The Buckets have a son whom we never see, but who has a habit of telephoning and always wants something from his mother or father, usually money. This son, Sheridan, is supposed to be at University (how old is he? If Richard is retired, how old were they when he was born. If he's at university he must be in his 20's. From what we discover, he comes over as incredibly camp. Is he gay? Does Hyacinth realise? Does his father? Again, like Miss Mountshaft, this invisible character helps to round out the characters of Hyacinth and Richard.
In 'One Foot In The Grave' we hear about the Meldrew's son, who died at an early age. Although not actually an invisible character, mention of this deceased child helps to give us more information about the back story of the Meldrew's. Although Victor is generally a somewhat irritating and annoying character, giving the background of their child does give them a bit more believability and makes Victor a good deal more three-dimensional and less of a comudgeon.