Meatball is a young, easygoing American hosting a massive, chaotic party that has lasted for almost two days. He is part of a hip circle of expatriates and intellectuals but remains mostly detached from their complex anxieties. Instead of panicking over the escalating disorder in his apartment, he adopts a stance of bemused resignation, making mild attempts to manage his unruly guests and fix his broken refrigerator.

Callisto is a middle-aged Italian intellectual and European expatriate who has transformed his apartment into a self-contained greenhouse and aviary. Haunted by memories of World War I and fearful of the looming threat of global destruction, he dictates his memoirs to his girlfriend while isolating himself entirely from the chaos of the outside world. He constantly monitors the outdoor temperature, viewing its stasis at 37 degrees Fahrenheit as an ominous sign of impending doom.

Aubade is Callisto's waifish, half-French and half-Annamese girlfriend. Described as a "tawny question mark," she lives in an isolated, intuitive state, feeling the concept of entropy physically rather than understanding it intellectually. She serves as Callisto's connection to the outside world, obsessively checking the temperature for him while trying to maintain the fragile equilibrium of their sanctuary against the disruptive noise from downstairs.

Sandor is a Hungarian expatriate and a guest at Meatball Mulligan's ongoing party. He is described as libidinous and suffering from a chronic case of Don Giovannism, quickly taking an interest in the uninvited female undergraduates who arrive at the gathering.

Saul is a disaffected American intellectual and a close friend of Meatball Mulligan. He arrives at the party distressed after a severe argument with his wife over communication theory, which leaves him convinced they will divorce. He expresses paranoid fears about government surveillance and human predictability to his host.

Miriam is Saul's wife, though she does not attend the party herself. She and Saul have recently engaged in a severe fight concerning communication theory, prompting a severe marital crisis.

Duke is the leader of the Duke di Angelis jazz quartet, an experimental musical group playing at the gathering. He takes avant-garde jazz to an extreme degree by instructing his musicians to silently mime their performance, claiming they are merely thinking the root chords of the song.