Aro is the leader of the Volturi. Behind his sophisticated, polite façade he hides deadly calculation and a thirst for power. Aro sees the war against the Cullens as a means to destroy their power by acquiring gifted family members, like Edward and Alice, for his own army.
Caius seems to be dead-set on forcing a fight against the Cullens. He believes that their claim that Renesmee is not an immortal child is a trick. Caius executes Irina in front of her sisters, Tanya and Kate, in the hopes of causing them to attack the Volturi, which would give them a reason to fight back.
Bella describes Marcus as always looking bored, as if he's just seen it all. When it comes to a vote between the Volturi brothers whether to fight the Cullens and their friends, he decides not to attack them.
Together with his sister, Jane, Alec plays a vital part as one of the Volturi's defense weapons, because he can cut off his opponents' senses, so they feel nothing.
Along with her brother Alec, Jane is one of the most valuable defense weapons in the Volturi guard, because she can inflict pain on others with her thoughts. Since Jane first met Bella, she has been trying to harm her, and now is furious when she realizes that she's no match for Bella's shield.
Chelsea is one of Aro's close personal guards, who has won many wars for the Volturi thanks to her ability to tighten or loosen the relationships between other vampires – for example, bonds of friendship and loyalty.
Demetri is one of Aro's guards because of his outstanding gifts as a tracker.
Felix is one of Aro's personal guards.
Renata is Aro's bodyguard. She's not physically strong, but has the power to shield Aro from others by diverting their attention away from him:
You simply find yourself going in a different direction than you planned, with a confused memory as to why you wanted to go that other way in the first place. She can project her shield several meters out from herself. (31.30)