Cassius, in Shakespeare's Julius Caesar, is the instigator in the plot to murder Caesar, which is introduced at the start of the play. He is the one that approaches Brutus to elicit his help as another conspirator. Brutus is a man who acts from a love of Rome, but Cassius seems to be motivated more by a political agenda and jealousy.
Act One, scene two, is known as the "seduction scene," when Cassius first approaches Brutus. He infers that Caesar is a danger to the state of Rome, but has no real evidence to support his claims. He comes off as a conniving, self-centered malcontent. Having once saved Caesar's life, it seems that he believes he should have been compensated more, and that his position within the empire should have been elevated. Instead he feels ignored by Caesar, acting like a spoiled child.
Cassius compares Caesar to a "god" (I.ii.116), while Cassius is nobody, despite the fact that Cassius saved Caesar from drowning:
...and Cassius is / A wretched creature, and must bend his body [bow] / If Caesar carelessly but nod on him (I.ii.116-118).
Cassius plans to send Brutus falsified proof to support his desire to see Caesar murdered. And though he repeatedly follows Brutus' lead in most of the decision-making, and expresses his love of Brutus, is this excellent "judge of men and their abilities" (Cassius) being honest, or simply trying to further his own ends? He is definitely not a man of nobility, dedication or loyalty.