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.
If Naruto was an Ajin in a fanfiction, the characters in Naruto would have a mix of reactions. Hinata would be concerned for Naruto's well - being. She might be a bit scared at first because of his new and strange abilities, but still stand by him. The villagers, who have had a rocky relationship with Naruto in the past, might be afraid of him all over again. They could see his Ajin powers as a threat. On the other hand, Naruto's close friends like Shikamaru would try to understand what's going on and help Naruto control his powers if they can.
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.
Sonic would probably be really curious about Naruto. He'd be intrigued by Naruto's ninja abilities, like his ability to run up walls or create clones. Naruto, on the other hand, would be amazed by Sonic's speed. He might try to keep up with Sonic at first, which would lead to some funny moments. Tails could be the one to explain to Naruto how things work in the Sonic world, while Naruto could share stories about his own world with the Sonic gang.
Sonic would be a main character for sure. Then there could be other Naruto characters like Sasuke and Sakura. Sasuke might be so focused on his revenge and training that he doesn't give Sonic a second thought. Sakura could be too caught up in her medical ninjutsu studies and her feelings for Sasuke. So Sonic is left in a world where these important characters are basically ignoring him, which makes for an interesting dynamic.
Obviously Naruto and Sonic are common characters as it's in the title. There could also be female characters like Sakura from Naruto series and Amy Rose from Sonic series.