In fiction, a foil is a character who contrasts with a major character to highlight good qualities of the major character. Cassius definitely contrasts with Brutus in several ways and in different situations throughout the play, highlighting Brutus's good traits.
How are these two men different?
Cassius fought in many battles and was considered to be a great soldier. Older than Brutus, Cassius initiated the plot to kill Caesar. Cassius disliked Caesar, thinking him weak and arrogant. Cassius' lack of respect and jealousy toward Caesar encouraged him to instigate the conspiracy. Now, a Roman Senator, Cassius cannot stomach the thought of Caesar being named Emperor of Rome for life.
Marcus Brutus is not a great soldier. He has spent his life working for the good of Rome. He is popular with both the people and the other Roman senators. Brutus epitomizes the thinkers of the day: he is sensitive, thoughtful, well-read, and torn between the good of Rome and the assassination of the man described as the greatest Roman of all time.
Contrast the two characters:
Cassius judges the character of men well. He wanted Antony dead alongside of Caesar because he knew that Antony would be dangerous.
Brutus does not. He believes that he can persuade Antony why the murder was committed.
For Antony is but a limb of Caesar.
Let us be sacrificers, but not butchers, Caius.
We all stand up against the spirit of Caesar,
And in the spirit of men there is no blood.
Cassius wants Caesar dead for personal reasons: jealousy, weakness, not deserving of the crown.
Brutus wants what is best for Rome. If Caesar has to die for it, then so be it.
Cassius makes decisions quickly.
Brutus is more thoughtful about his choices.
Cassius needs Brutus to make the assassination more palatable. He is willing to manipulate Brutus and do whatever he needs to do to win him over.
Brutus needs the other conspirators when he decides to become a part of the plot. He trusts Cassius and would have been surprised to know that he was tricked by him.
The characters of Gaius Cassius and Marcus Brutus find themselves forever linked. Brutus was the brother-in-law of Cassius. In addition, they both became leaders in the plot to kill Caesar.
Both characters play an important part of the most despicable assassination in history.
Shakespeare's Julius Caesar takes place in 44 B.C. and is based on actual historical events. All of the Dramatis Personae were real people in history.