The title of Caesar is bestowed upon the rulers of Imperial Rome. It is derived from the cognomen of Gaius Julius Caesar, who effectively transformed the Roman Republic into an Empire. He is one of the few personas in the series that is based on an actual person rather than a mythological figure (Julius Caesar was deified by the Roman Empire however and was supposedly killed by Brutus and replaced by his adopted son Octavian, later known as the Emperor Augustus).
Like Akihiko Sanada's initial persona, Polydeuces, he was a great warrior who ascended to become a god upon his death. Its appearance seems to be a mix of the colors of Castor and Polydeuces blended together. Augustus was known as the "ruler of the world", which is represented by the globe carried in Caesar's hand. The little man inside of his chest is a reference to Akihiko's admiration of Shinjiro, whom he viewed as a role model. Even after his demise, many people still viewed him as insignificant, but he meant the world to Akihiko as one of the biggest influences in Akihiko's life.
The classical Latin pronunciation of the name is (kae-sar), emphasizing the diphthong "Æ". In the English version of Persona 3, Akihiko's pronunciation when he summons this Persona softens the diphthong and is pronounced as (see-zer), its Anglicized variant.
The little man's red mask inside of Caesar bears a striking resemblance to Castor's own mask.