Cassius and Brutus are both aristocrats. They are friends and have known each other all their lives. Brutus says of Casca:
What a blunt fellow is this grown to be!
He was quick mettle when he went to school. (I.2)
All three men went to school together as boys.
Cassius and Brutus prove themselves to be courageous, both earlier in the play when they assassinate Caesar, and later at the Battle of Philippi, where they fight to the bitter end and then commit suicide to forstall capture.
Both men are obviously intelligent and perceptive. This is first shown in Act I, Scene 2, when they are conversing about the present turbulent state of affairs. For example:
What means this shouting? I do fear the people
Choose Caesar for their king.
Ay, do you fear it?
Then I must think you would not have it so.
Both Brutus and Cassius are proud men. Cassius is much more free about expressing his pride, but Brutus displays his own pride many times throughout the play, including in the speech he makes to the plebians right after the assassination. Brutus is more proud of his family honor than Cassius, who is proud of his own intelligence, knowledge, and independent spirit; but both are proud men in their own ways.
Both men are proud of their skill as soldiers. This becomes a source of conflict betweeen them when they are facing a climactic battle with the forces of Antony and Octavius at Philippi. Both Brutus and Cassius are exceedingly proud to be Romans.
You say you are a better soldier.
Let it appear so, make your vaunting true,
And it shall please me well. For mine own part,
I shall be glad to learn of noble men.
You wrong me every way, you wrong me, Brutus.
I said an elder soldier, not a better.
Did I say better?
If you did, I care not.
Shakespeare had to give his main characters distinguishing characteristics in order for the audience to be able to tell them apart. The play only lasts a couple of hours and is full of men dressed up to look like what it was thought the ancient Romans looked like. It is noteworthy that the characters are always addressing each other by name, helping the audience to keep their identities straight. Caesar, for example, is always calling himself Caesar.
Shakespeare made Casca a "blunt fellow." He made Antony an athelete and a hedonist. He made Octavius a hotheaded, impetuous youngster. He made Julius Caesar an egomaniac. He made Cassius a miser and prevaricator, and he made Brutus a studious, philosophical type of person, not unlike Hamlet.