Antony is stubborn and ambitious.
Ambition is a common trait among the play’s characters, but it does not turn out well for any of them. Antony wants power, and will do anything to get it. He was Caesar’s right-hand man, and he believed that aligning with Caesar would increase his chances of advancement. Even after Caesar died, he did not give up.
In the beginning of the play, Antony is Caesar’s flunky. He does whatever Caesar wants, as is demonstrated when Caesar asks him to touch his wife during the race on the Feast of Lupercal. When Caesar dies, Antony is horrified. He sees his chances of advancement dying with him.
Antony demonstrates his stubbornness when he makes a prescient announcement after finding Caesar’s body.
And Caesar's spirit, ranging for revenge,
With Ate by his side come hot from hell,
Shall in these confines with a monarch's voice
Cry 'Havoc,' and let slip the dogs of war;
That this foul deed shall smell above the earth
With carrion men, groaning for burial. (Act 3, Scene 2)
Antony is willing to throw Rome into civil war to prevent the assassins from taking charge of Rome.
Antony takes charge after Caesar dies. He does not just disappear into the woodwork, as Brutus predicted. Instead he confronts Brutus and his cronies, and asks to speak at Caesar’s funeral. This is incredibly ambitious and cunning. Antony knows that he can sway people’s opinions of Caesar, and of Brutus.
Antony’s speech is successful. The people of Rome turn on Brutus and the others, and Antony is in charge. This is short-lived though. To keep power, Antony needs allies. His biggest threat is Octavius, Caesar’s nephew and heir. Antony forms a triumvirate with Octavius and the leader of the other big army, Lepidus.
Antony’s skill at manipulation is no match for Octavius’s. Although Antony was able to delay Octavius long enough to make his speech, he had to work with the young man eventually. Part of the process of the triumvirate is proscription, the killing of enemies or taking of their property. When Lepidus comments that one of the condemned is Antony’s sister’s son, Antony’s response demonstrates his ruthlessness.
He shall not live; look, with a spot I damn him.
But, Lepidus, go you to Caesar's house;
Fetch the will hither, and we shall determine
How to cut off some charge in legacies. (Act 4, Scene 1)
Ruthlessness aids ambition, and so does stubbornness. Antony is able to stay one step ahead of Octavius or at least neck and neck with him until after they defeat Brutus and Cassius. From that point on it will be a battle between the two of them, and eventually Octavius will win. Ambition and stubbornness are no match for brilliance.