Cassius is deceitful. This is apparent throughout his conversations with Brutus to get him to join the conspirators, but it is most specfiically seen when he tells other conspirators to write letters to Brutus saying how badly they want him to overthrow Caesar and has them forge citizens' names to the letters.
Cassius is also emotional and hot tempered to the point of being irrational. This is best evidenced when he and Brutus argue in the tent in Act IV. Brutus accuses him of being corrupt since the assassination. The argument reaches a climax when Cassius presents his sword to Brutus and tells him to kill him if he believes he's acted unjustly.
Cassius is also wise. He had some great ideas that could have made both the assassination andthe war verse Antony turn out differently, but Brutus vetoed Cassius' idea and things quickly went awry. Two examples are when Cassius foresaw the danger Antony could present, so he suggested killing him along with Caesar. The other was when he told Brutus to let the enemy come to them instead of vice versa. If Cassius' wise recommendations would have been heeded, the history of Rome would likely have been written much differently.