The Brás Cubas of the novel is a character invented by Machado de Assis. But the name belongs to a real man: Brás Cubas, a Portuguese nobleman of the sixteenth century, captain-major of the captaincy of São Vicente, to whom the founding of the town of Santos is attributed, and who died in 1592.
The book itself treats this coincidence in chapter III, "Genealogy". The narrator recounts that the family's true founder was a cooper named Damião Cubas, and that the narrator's father, great-grandson of Damião, ashamed of a surname that smelled too much of cooperage, first tried to connect himself to the family of the captain-major, "that famous homonym of mine", and therefore gave his son the name Brás. The captain-major's family refused the kinship, and the father then invented that the surname came from a knight who had seized three hundred tubs from the Moors.
The episode is the first joke in the book about the family's vanity: the name of the historical hero serves to disguise a humble origin, and the narrator, already dead, dismantles the sham without ceremony.