Milo, better known as Caesar, is the son of talking chimpanzees Cornelius and Zira, in the Planet of the Apes movie series. He is the leader (and chief instigator) of the ape revolution in the fourth entry, Conquest of the Planet of the Apes, and the ruler of Ape City in the fifth and final film, Battle for the Planet of the Apes. He was portrayed in both movies by actor Roddy McDowall, who had also played Cornelius.
Originally named Milo after Dr. Milo, who travelled back in time with Cornelius and Zira to the Earth of Taylor's era, he was reared by his human foster father Armando, a traveling circus owner who gave Cornelius and Zira sanctuary when Zira went into labor in the final act of Escape from the Planet of the Apes. Before departing, Zira switched the infant Milo with a young, chimpanzee recently born to Armando's primative chimpanzee. The other chimpanzee baby was killed with Cornelius and Zira by the human, Dr. Otto Hasslein. Milo speaks his first word, "Mama", at the very end of the film. His false identity secure, Milo grew up as a mute acrobat.
Armando addresses Milo/Caesar by the name Caesar in the next film, Conquest of the Planet of the Apes.
In the opening minutes of Conquest, he and Armando are visiting a large city for the first time, and Armando leads him around on a chain. In private, though, Caesar stands nearly erect, and chats back and forth with Armando like a father and son. Armando critiques Caesar's "act" of behaving like a primitive chimpanzee, then gives him an idea of what to expect: In the years Caesar has been growing up in the provinces, many of the events his parents warned humanity about (before they were murdered, in an attempt to prevent their occurrence) have taken place – the big one being the spaceborn plague that killed virtually all of Earth's dogs and cats, leaving humanity without pets. Monkeys at first took their place, then gradually apes, who progressed (so to speak) from companions to household helpers – to their present state of slavery.
Caesar tries to take what he sees (groups of apes being dispersed, chimps and orangutans being scolded or punished for honest mistakes – or for exhibiting apelike behaviour) in stride, but when he sees a gorilla first beaten by police, then shocked with a cattle prod, he impulsively cries out "Lousy human bastards!" When the police turn their attention his way, Armando insists that he was the one who spoke, but other people nearby swear differently. Caesar panics and runs away, leading them to suspect he understands what's going on – and perhaps he can speak.
Armando is released, and catches up with Caesar momentarily, but knows now their charade (that Caesar is a mute, primitive ape) might have failed. He instructs Caesar to hide among his own, and join a shipment of apes brought in by boat, if Armando can't convince the authorities that he's harmless – and only ran away because of the commotion. Armando goes downtown, to try to bluff their way out of trouble. When he fails to return, Caesar infiltrates a shipment of apes.
Passing his conditioning with flying colours, Caesar is next sold to Governor Breck, supervised by his assistant Mr. MacDonald, ironically an African-American descended from slaves. When Breck decides to formally name him, he takes out a reference book, turns to a page and points to a name at random, then bids Caesar to do likewise. Caesar chooses his adopted name, pretending to do so randomly, and is so registered. Next he is assigned to the city's "command post" – the communications centre for Ape Management, and its lockup for disobedient apes. (He is also selected to mate with Lisa, presumably resulting in the birth of their son Cornelius, who appears in the next movie.).
When Caesar learns that Armando died while in custody, he decides enough is enough, and begins plotting an ape revolt, conspiring with other apes and driving them to turn on their masters. When Caesar is belatedly traced and discovered not to have been part of any ape shipment, he is captured and tortured by Breck, to see if he is indeed the talking offspring of two talking apes. MacDonald excuses himself from the scene, and changes the breaker settings for the electroshock table Caesar is wired to. He can't prevent Caesar from being shocked to where he finally chokes out the words "Have pity!", but he can prevent Caesar from being electrocuted.
Believed dead, Caesar kills the handler assigned to dispose of him, then throws the switch that opens all the cages in the ape lockup, and the revolt begins. Hours later, much of the city is in flames, the police and military have been beaten down, and the apes are in control, as Caesar predicts will follow around the world when word spreads. MacDonald tries to dissuade Caesar from further violence, while Lisa becomes the next ape to speak, telling Caesar "No!" when he condemns all humanity.
In the fifth and final movie, Battle, human and ape children gather around a statue of Caesar, now a legend who's been dead for six hundred years, as the Lawgiver tells them the story of how Caesar fought another battle, 27 years after the first, that both solidified his position as ape leader, and convinced him to give a joint ape-human society a chance, instead of one species dominating the other. The Lawgiver earlier in the series had laid down the command that humans were to be shunned and driven out; the sight of him regarding the children of both species alike at the end, is persuasive that the timeline had indeed changed, and Caesar had given the world a better future--assuming that the Lawgiver's teachings were not distorted in later generations. This seems likely given screenwriter Paul Dehn's statement (as cited in the book "Planet of the Apes Revisited" by Joe Russo on page 211) that the tear on Caesar's statue at the end of the film was to tell the audience that Caesar's efforts ultimately failed, as mankind fell and the society seen in the first two films was born.