Off-Stage Characters And A Writing Career

Yamamura's wife is mentioned again in Murder Bound but is she one of those fictional characters who remain permanently off-stage and how many of those are there? Sometimes in a play, the characters expect another character but we know from the cast list that he will not arrive.

Is Richard Telford a character in the novel? He remains off-stage but is mentioned twice and described once.
-copied from here.
 
(And the author, CS Lewis, puts Telford to good use.)

Coronation Street had a handful of permanently off-stage characters and then, I was told, one of them appeared.

The blurb for Poul Anderson's first detective novel, Perish By The Sword, describes the author as:

"Already well established as a science fiction writer..."

- whereas the blurb for his third detective novel, Murder Bound, describes him as:

"A familiar figure in the mystery field..."

- but then he found that he could earn more as an sf writer!  

But he could also write richer and more varied narratives in sf. This blog would not exist if Anderson had written mostly mystery novels.