Here is a very brief sketch of the characters you're interested in. You can find much more complete information by visiting the "Character Analysis" page here at eNotes (links below):
Mark Antony is loyal, emotional, and a skilled rhetorician. His speech at Caesar's funeral turns the riotous crowd against the conspirators.
Brutus "plays a prominent role in the conspiracy against Caesar. The primary issues surrounding Brutus's character are his idealism and devotion to the principle of republicanism, his political judgment, his motives for joining the conspiracy, and his role as a tragic hero."
Portia is the devout and sensitive wife of Brutus. She argues with him that a husband and a wife should share everything. To prove her devotion to him later on, she wounds herself in the thigh.
Calpurnia is Caesar's wife with a sensitivity towards omens. She is somewhat persuasive with her husband, but ultimately unsuccessful with her warnings.
Cassius and Casca are the conspirators who act to overthrow Caesar. Cassius is politically motivated, and helping him in his goals are his innate ability to assess men and their use to him. Casca is one of those Cassius targets as useful. Casca is bitter and cruel, ridiculing both Caesar and the crowd.