Captain America might be paired with Ultron as well. Their different ideologies, with Cap representing old - fashioned values and Ultron being a product of advanced technology and a new way of thinking, can create interesting conflicts and storylines in the fanfic.
Ultron is obviously one of the main characters. He's the central villain. Then, there are likely to be Avengers characters like Iron Man, as he has a connection to Ultron's creation. Captain America might also be there, representing the moral opposition to Ultron's evil plans.
Perhaps a group of anti - technology extremists could be the main antagonists in an Ultron fanfic. They see Ultron as the symbol of all that is wrong with technology and are determined to take him down, no matter the cost. They might use their own advanced, but more rudimentary, technology to fight against Ultron. This would create an interesting contrast between Ultron's high - tech abilities and their more basic, yet effective, methods.
The main characters are likely to include superheroes like Iron Man and The Avengers. Since Ultron is the main villain, they would be the ones to oppose him.
In many Tony Stark and Ultron fanfictions, Pepper Potts can be a great character. She provides a human connection for Tony and can often be the voice of reason. Also, Captain America can be a great addition. His moral compass clashes with Tony's more pragmatic approach, especially when it comes to dealing with Ultron. This creates great drama and tension in the stories.