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Cassius is the ringleader of the conspirators. He's politically savvy and manipulative, and he absolutely resents the way the Roman people treat Julius Caesar like a rock star. More important, he hates the way Caesar runs around acting like a god: "Why, man, he doth bestride the narrow world /Like a Colossus, and we petty men / Walk under his huge legs and peep about" (1.2.142-144).
Cassius is also responsible for manipulating Brutus into joining the conspiracy (although Brutus may have already been thinking of turning against Caesar):
Well, Brutus, thou art noble. Yet I see
Thy honorable metal may be wrought
From that it is disposed. Therefore it is meet
That noble minds keep ever with their likes;
For who so firm that cannot be seduced? (1.2.305-309)
Bragging to the audience, Cassius compares himself to a metal-worker as he suggests that even the noblest of men can be manipulated, or bent, to his will. How does Cassius "seduce" Brutus? First he slyly suggests that the Roman people want Brutus to lead them, then he sends Brutus some forged letters urging him to take down Caesar.