Maybe there's a mysterious figure from Professor Pickle's past. This antagonist has some unresolved issues with him and sees Amy as a way to get back at him, so they start causing trouble for the two.
It could be a jealous colleague of Professor Pickle. This person might be envious of the time and attention he gives to Amy and tries to sabotage their work and relationship.
Perhaps they are both part of a scientific research project. Professor Pickle is the experienced mentor and Amy is the enthusiastic newbie. They face various challenges during the research, like equipment failures and unexpected results, which bring them closer.
It could be Voldemort, of course. He would be a formidable foe, and it would be interesting to see how Amy and Rory's knowledge and skills would help Harry in the fight against him.
Another option is that Metal Sonic could be the main antagonist. He might see Amy Rose's new bad side as an opportunity to create chaos and undermine Sonic. Metal Sonic could manipulate Amy Rose further into doing more evil deeds, all while trying to frame Sonic for her actions.
Another option is that it could be an existing character from their respective universes who has turned against them. For example, one of Lord Boxman's former robots that has gained sentience and now wants to destroy him and anyone associated with him, including Professor Venomous.
It could be Mystique. She's always been one to cause trouble for the X - Men. Maybe she wants to disrupt the X - Men's operations by making Professor X and Storm swap bodies. She could use their confusion to her advantage, like stealing important information or causing internal strife among the team.
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.
Definitely Ty Borden. He and Amy have a special relationship, so he would be deeply concerned about her illness. He could be the one running errands, getting medicine and making sure she has everything she needs. Georgie could also be a main character. She could be helping Lou with the ranch work and also spending time with Amy, telling her stories to keep her spirits up. And then there's Spartan, Amy's horse. Spartan could be acting strangely because he senses something is wrong with Amy, and his behavior could be a part of the story as well.
The main characters are obviously Amy and Tails. Amy is the one doing the comforting, and Tails is the one being comforted.
Another possible main character could be her rival. This rival could be from a different boxing school or have a completely different boxing style. Their interactions both in and out of the ring would be a major part of the story.
There might be a mysterious stranger. This person could be passing by when they hear Amy Fleming's screams for help. They could have some special skills or knowledge that can assist her in whatever situation she's in. Another possible character could be an authority figure like a sheriff or a teacher, depending on the setting of the fanfic, who has the power to protect her.