Wiktionary
n. Any fictional character drawn from some stereotype who is instantly recognizable.
Wikipedia
A stock character is a stereotypical person whom audiences readily recognize from frequent recurrences in a particular literary tradition. Stock characters are archetypal characters distinguished by their flatness. As a result, they tend to be easy targets for parody and to be criticized as clichés. The presence of a particular array of stock characters is a key component of many genres.
Usage examples of "stock character".
I didn't see any corpses or even collapsed drunks for the first fifty paces, though a man in a doorway tried to sell me a dagger and a mustachioed stock character offered to fix me up with something young and tight.
Actors deficient in observation and resource adopted this stock character of the Lawyer, and he was to be seen in hundreds of plays.
David Selig, the telepath, is no stock character from the rag-bag of sf stereotypes.
So, in Perseus we had that stock character: the uppity slave who abuses his position - and who somehow gets away with it.
Perhaps I should say that the real origin of the name and title is a stock character from Italian comedy-a buffoon or a fool.