Antony is one of the more intriguing characters of the play. We see evidence of several different sides to him. At first he appears in rather a slight role and somewhat servile in front of Caesar, but following Caesar's assassination he comes into his own.
First, he acts resourcefully in meeting with the conspirators immediately following the assassination, fooling Brutus with his show of meekness, and extracting from him the promise that he can speak at Caesar's funeral - an opportunity which he turns to his advantage. He is quick to exploit situations for his own benefit; he is proactive, and extremely subtle and manipulative in his handling of the crowd at Caesar's funeral. In all these traits, he is the opposite of the idealistic and somewhat impractical Brutus - as Brutus himself notes.
I am not gamesome; I do lack some part
Of that quick spirit that is in Antony (I.ii.28-29)
Antony, then, is 'quick' in thought and action and deed while Brutus ponders, reflects, and very often agonises over his decisions.
Antony however also reveals himself to be a cold and calculating type. He plots the downfall of his enemies, has dozens of senators put to death, and looks to seize power along with the equally ruthless Octavius while getting rid of their less effective companion Lepidus. He is instrumental in steering Rome to civil war and plans to gain personal profit from the situation.
Along with all this, though, Antony does display genuine emotion at Caesar's death and vows to avenge him. Although he can personally gain from the power void left at Caesar's death, when left alone with the body he laments his friend's passing rather than speculating upon his own plans for power. In his most private moments, therefore, he appears as a devoted friend rather than being just concerned with himself.
Antony, then, is a varied and somewhat contradictory character. He proves himself a loyal friend to Caesar, an astute politician, and also a ruthless power-seeker in his own right.