The pirate who becomes an Assassin in Assassin's Creed IV: Black Flag is Edward Kenway.
Edward is a Welsh privateer and pirate active during the Golden Age of Piracy. At the beginning of the story, he takes the clothing and identity of the Assassin Duncan Walpole after Walpole's death. Edward initially sees the disguise as a way to gain money and influence, rather than as a commitment to the Assassin cause.
His choices bring him into contact with figures such as Edward Thatch, Benjamin Hornigold, and the Assassin mentor Ah Tabai. Over time, Edward's pursuit of wealth exposes the destructive consequences of the pirate world and changes his attitude toward loyalty and responsibility.
Edward Thatch is better known as Blackbeard and is a major historical character, but he is not the game's protagonist. Adéwalé, Edward's quartermaster, later becomes the protagonist of the Freedom Cry expansion.