To what does the rhetorical device ethos refer?

Ethos refers to an appeal to ethics.

Explanation:

Ethos means character and comes from the word ethikos, which means moral and moral personality. For speakers and writers, ethos is formed by the call to ethics and credibility and identification of the speaker with the audience. The speaker must be trusted and respected as an expert in the subject under review. For arguments to be effective, it is not enough to make logical reasoning. Content should also be presented efficiently to become trustworthy.

the characters in string all have major secrets that they strive to avoid revealing throughout the play.