What is one universal theme illustrated by the character relationships in endgame?

Endgame is a story written by Irish novelist Samuel Beckett as a "one act play" laid down around a "not fully explained" apocalyptic disaster comprising 4 characters.

The main universal theme illustrated by the characters relationships in the story is "Power Dynamics", and particularly power dynamics among humans.

Social power is meant to be the ability to influence people's behavior without the use of force or coercion, although sometimes people abuse power;.

Since human relationships very often involve issues of power and control, relationships among 2 or more people is a dynamic process; hence social power dynamics are the ways how people interact with each other while certain individuals have more power than others.

Endgame melts power dynamics with tragedy and comedy contributing to the philosophical movement around Beckett's time (Theater of the Absurd) or "Absurdism", conveying the meaningless of life and stories beyond human rationality.

greet them:

buenas noches.

that is considered night time which is when "good night" is used.

consonance, it has the repetition of the consonants in stressed syllables with different vowel sounds.