What character does Brutus think is weak and powerless but turns out the opposite?

Brutus initially perceives Cassius as weak and manipulative, believing that he lacks the moral integrity to be a true ally. However, as the conspiracy against Caesar unfolds, it becomes clear that Cassius possesses significant influence and cunning, proving to be a formidable player in the political machinations of Rome. This misjudgment highlights Brutus's naivety and idealism in the face of the harsh realities of power.