What are the good and bad character traits of Brutus and Cassius in Shakespeare's Julius Caesar?
As the other educator points out, Cassius has few positive qualities. In fact, he is altogether manipulative, dishonest, and corrupt.
In Act 1 Scene II, Cassius uses flattery to manipulate Brutus. He tells Brutus that the public reveres him (Brutus) beyond imagination. Cassius asserts that Brutus has thoroughly underestimated his own popularity; the wily senator offers to be a mirror by which Brutus could ascertain his true worth.
Cassius then tells Brutus tales about Caesar's apparent physical infirmities; he portrays the emperor as a frail man—one unfit to "get the start of the majestic world / And bear the palm alone." Cassius is determined to use subterfuge to manipulate Brutus and to turn him against Caesar.
At the end of Act 1 Scene II, Cassius acknowledges that Brutus is an honorable and principled man. Yet, he is confident that Brutus can be molded and influenced against his better judgment:
Yet I seeThy honorable mettle may be wroughtFrom that it is disposed. Therefore it is meetThat noble minds keep ever with their likes,For who so firm that cannot be seduced?
Brutus is a good leader, he is truthful, he is respected by other members of the senate and the public, and he is a man of conscience. He is a skilled orator, though not as skilled as Antony. He has what's best for his country in mind. He takes a while to make decisions, which is indicated in the play by his inability to sleep well with the decision to kill Caesar or not weighing heavily on him. He is a good husband and treats his wife as an equal most of the time.
However, he is gullible. He allows himself to be mislead by planted letters written by other conspirators. He does not confide in his wife while he is trying to make his decision. He does not pay attention to the other harbingers of evil doing--the weather, for instance. He misjudges Mark Antony and seals his fate by allowing Antony to speak to the Roman people regarding Caesar.
There are not as many positive qualities for Cassius. Even Caesar is leery of him when he says, "he has a lean and hungry look". Cassius is ambitious to a fault. He does not want Caesar to rule Rome, but rather wants to be part of the ruling party. He manipulates the honorable Brutus to take their side so that the conspirators will look better in the eyes of the public after the deed is done.
Cassius does have good judgement of character, and he proves this by choosing Brutus as a recruit for their task. He also is partially correct about Caesar's ambition. He does also prove to be loyal to Brutus on the battlefield.