Sonic would probably be very shocked at first. He trusts Amy, so her betrayal would be unexpected. He might initially try to talk to her to understand why she did it.
Tails would probably be very curious and eager to figure out how to change Sonic back. He might study Sonic's new form scientifically. Knuckles, on the other hand, would be a bit more confused but still stand by Sonic's side. Amy might be initially jealous because she's used to being the main female around Sonic, but then she could become a great friend and help Sonic adjust. Dr. Eggman would likely see this as an opportunity to take advantage of Sonic's confusion in her new form.
The characters in One Piece and Sonic are very different. Sonic is a heroic hedgehog from a modern - ish world, while the One Piece characters are pirates, marines, and various other fantastical beings. When they meet, there could be a lot of misunderstandings at first. For example, Sonic might not understand the concept of the Pirate King right away. And the One Piece characters might be freaked out by Sonic's non - human appearance. But as they get to know each other, they would find common ground. Sonic could help the Straw Hat Pirates in their adventures with his speed, and the pirates could introduce Sonic to the wonders of their world, like the different islands and the secrets of the Grand Line.
Sonic might be initially shocked and a bit confused. But being the hero he is, he would probably give her a chance to prove herself. Tails would be very curious and might try to figure out how the transformation happened scientifically. Knuckles would be more guarded, not trusting her right away. Dr. Eggman, on the other hand, could be either furious if it wasn't part of his plan or intrigued if he sees potential in this new form for his evil schemes.
Sonic might be really confused if he is under the effect of the love potion. He's always been independent and suddenly having strong romantic feelings for Amy would be a big change. He might try to fight the feeling at first because it's so alien to him.
Tails would probably be very helpful. He'd use his intelligence to make sure Amy has a comfortable pregnancy, like creating special devices for her. Knuckles might be a bit awkward at first but would ultimately be very protective. He'd stand guard to make sure no harm comes to Amy and the unborn baby.
Well, Sonic is a hero at heart. So when Amy gets hurt, he would first assess the situation calmly, but inside he'd be worried. He'd make sure she was stable and then look for the cause of her injury. If it was an enemy, he'd go after them. But he'd also be very gentle with Amy, talking to her softly and reassuring her that she'll be okay. He'd do whatever it takes to see her back on her feet again, like finding special herbs or items that could speed up her healing process.
Some fans might be very shocked and disappointed. Sonic and Amy have a long - established relationship in the Sonic franchise, and this would be a huge deviation from what they expect.
Well, Sonic is the central figure. But other characters like Amy Rose could be important. She might be heartbroken to see Sonic turn dark and try to use her love for him to bring him back. There could also be some new characters created specifically for this fanfiction. For example, a mysterious character who has the power to either make Sonic darker or has the key to bringing him back to normal.
The main characters are probably Sonic and an alternate version of Sonic. There could also be other Sonic characters like Tails or Knuckles making appearances to support the Sonics in their rescue mission. But Sonic and his doppelganger would be at the forefront, using their speed and skills to achieve the rescue.