A believer in Saint Francis of Assisi, "who loved all animals", the jovial, warm-hearted Armando readily comes to the aid of Cornelius and his pregnant wife Zira during Escape from the Planet of the Apes, when most of humanity has rejected them, and even the United States Government plans to prevent their ever having children, since they someday "may constitute a threat to the human race." While Armando does his best to help the pair, and conceal their newly-born son Milo (later called Caesar), he can't prevent their murder at the hands of Dr. Otto Hasslein. He does, however, take responsibility of Caesar, raising him publicly as a circus chimpanzee, while privately teaching him human knowledge and introducing him to human habits.
In the fourth Apes movie, Conquest of the Planet of the Apes, Armando brings Caesar to a large city for the first time, after raising him mostly in "the provinces" and remote areas, where Caesar has been unaware of apes' adoption as pets (to replace the dogs and cats lost to a spaceborn plague), which has led gradually to their present enslavement. Armando keeps Caesar on a short leash in public, while coaching him on apelike mannerisms when nobody's looking. All the same, fate gets the better of them; when Caesar sees an ape (the gorilla Aldo) mistreated, he swears his frustration – bringing himself and Armando to the attention of the police.
Slipping away, Armando and Caesar try to formulate a plan to prevent further trouble. Armando already claimed to be the one who cried out, and plans to tell the authorities that Caesar is frightened of cities, and ran away because of the public commotion. He tells Caesar to wait for him near the docks where shipments of apes arrive nightly for "conditioning" and training as servants, and to infiltrate a shipment if Armando fails to return. Caesar does so, finding himself by degrees sold to Governor Breck, and assigned to Ape Management's command post.
Armando undergoes a lengthy, tense interrogation at the hands of the state's authorities, who ultimately believe his alibi, and that Caesar is mute and not the son of "the talking apes". They insist, though, that he pass a screening by the "Authenticator" – a hypnotic device which compels its subjects to tell the complete truth – before he is released. Armando first refuses to submit on gentlemanly grounds, then when he sees no other way out, he throws himself through a window (falling to his death) rather than reveal what he knows about Caesar.
When Caesar learns of Armando's death, it becomes the last straw for him, and he begins to perpetrate an Ape revolt, executed over the rest of Conquest.