Sonic would be devastated. He and Tails are like brothers. Sonic would likely go into a rage and vow to avenge Tails' death. He would immediately start hunting down whoever or whatever was responsible. Amy would be heartbroken as well. She would probably be the one to try to comfort Sonic while also being deeply sad herself. Knuckles would be angry and ready to fight alongside Sonic to seek justice for Tails.
Sonic would be in shock at first. His best friend and sidekick is gone. He would feel a deep sense of guilt, thinking that he should have been there to protect Tails. This could lead him to become more reckless in his pursuit of the killer. Amy would be crying uncontrollably and would try to organize a proper memorial for Tails. Knuckles would be solemn and would use his strength to protect Tails' legacy. He would also be on high alert, fearing that the same fate could befall other friends.
Sonic would be consumed by grief and anger. He might isolate himself for a while to come to terms with his loss, but then he would emerge with a new determination. He would start gathering information and clues about Tails' death, using his speed to cover large areas. Amy would be in a state of mourning and would try to keep the group together during this difficult time. Knuckles would be seething with rage beneath his tough exterior. He would be ready to use his fists to bring those responsible to justice and would not rest until they were punished.
His friends would be devastated. Scott, for example, might go into a state of extreme anger and blame himself for not being able to protect Stiles. Lydia could be inconsolable, using her banshee powers to try and find some connection to Stiles in the afterlife.
Sasuke would be extremely enraged. He has a deep bond with Naruto, and Naruto's death would likely send him on a quest for revenge against Konoha. He might see it as a betrayal of everything Naruto stood for and fought for.
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.
Yamamoto would be more level - headed. He would first try to find out the details of Tsuna's arrest. He might talk to the police or other authorities to see if there's been a misunderstanding. He'd also be a source of support for Tsuna's family during this tough time.
Sonic would probably be very confused at first. He might mistake Tails in Amy's body for the real Amy and be really surprised by her different behavior. Knuckles would likely be suspicious and keep asking questions. Cream might be worried about her friends and try to help them figure out what happened. Big the Cat probably wouldn't notice much difference at all, just going about his normal fishing routine.
Sonic would be the first to react. He'd be angry and immediately start the search for Tails. Amy would be worried sick and might try to help Sonic in any way she can, like gathering information from the other animals in the area. Knuckles would be on high alert, thinking this could be part of a bigger plan to get to the chaos emeralds. He'd guard the emeralds even more closely while also keeping an eye out for any signs of Eggman's activities related to Tails' kidnapping.
The world would likely be in chaos. Sonic is often the main defender against villains like Dr. Eggman. Without him, Eggman could easily start taking over cities and enslaving the population. His friends would also be in a state of shock and grief, which could temporarily paralyze their ability to act.
His friends would most likely be very concerned. They would probably bring him things like food and medicine. TK might pray for his quick recovery.
Fishlegs would be really concerned and would use his knowledge to try and help. He might look for herbs or remedies that could aid in Hiccup's recovery. Snotlout, on the other hand, might initially try to act tough but deep down he'd be just as worried. He might try to show off his strength by helping to carry Hiccup or something like that. And Tuffnut and Ruffnut would be their usual chaotic selves, but also very caring towards Hiccup in their own unique way. They could make jokes to try and cheer him up while he's in pain.
Naruto would be shattered. Given his strong sense of justice and his connection to Hinata, whether it's a romantic or platonic one. He would initially be numbed by the shock. But soon, his emotions would erupt. He could go through different stages, from mourning and self - blame (thinking he could have protected her) to a fierce determination to make things right. His actions would be driven by his love and respect for Hinata, and he would not rest until he has either avenged her or found a way to bring her back, if possible.