Antony opens "Julius Caesar" as something of a wild child, during the feast of Lupercal. He is clearly a favorite of Caesar, as he is asked to touch Julia, the wife of Caesar, so that she might conceive. While the conspirators realize he is a man of importance to Caesar, they underestimate him, and he proves to be more than a mere limb of Caesar, who can be rendered powerless with out the figurative head of power. This proves to be fatal mistake in the end. We see Antony come into his own during the funeral speech. As power is divvied up, Antony claims what he feels is his, putting him at odds with Octavian, the young, adopted nephew of Caesar.
Antony is see as incredibly loyal to Caesar; in fact, Cassius suggests killing Antony because Antony is so loyal to Caesar that he might seek revenge. Antony is referred to in the play (and was historically) a partier and a playboy. During Act III, Antony uses his intelligence and power of speech (rhetoric) to sway the commoners to riot and drive Brutus and Cassius out of town, seemingly to avenge Caesar's death. At the beginning of Act IV, Antony has changed from a man who is looking out for the plebians and his friend to a war general, looking to cut the plebians off from Caesar's inheritance. Antony is either very changeable or two-faced--from the historical notes, we can assume he is two-faced and more interested in personal gain than the good of the Republic.