Final answer:
Westley becomes the Dread Pirate Roberts after being captured by the previous Roberts, who trains him and then selects him as a successor to maintain the feared persona.
Step-by-step explanation:
The character Westley becomes the Dread Pirate Roberts in William Goldman's novel "The Princess Bride", which was later adapted into a popular film. In the story, Westley is captured by the Dread Pirate Roberts while seeking his fortune to marry his beloved Buttercup. Instead of killing Westley, the Dread Pirate Roberts takes him under his wing, and after years of working together, Roberts selects Westley as his successor. It's revealed that the title of Dread Pirate Roberts is a persona that has been passed down from one individual to another to maintain the fearsome reputation, with each new Roberts retiring and selecting a successor. Westley inherits the title when his predecessor retires.