Sam would probably be really intrigued. She might see it as a cool new form for Danny and would be eager to learn more about how it affects his powers. Tucker, on the other hand, would likely be more concerned about how to reverse it at first. But once he realizes that Danny can still do some cool things as a mermaid, he'd be excited too.
His friends would initially be very surprised. Sam would be the first to try and comfort Danny and assure him that they'll figure out a way to change him back. Tucker would start looking into any technological solutions. Jazz would offer her emotional support and also try to analyze the situation from a scientific and psychological perspective. As time goes on, they would all come to accept Danny's mermaid form and help him use it to his advantage in dealing with any threats, whether they be on land or in the water.
His parents might be really confused at first. They've been hunting ghosts, and suddenly their son is a baby again. They'd probably be worried about his well - being and try to figure out what happened using their scientific knowledge. Jazz, his sister, would likely be very protective. She'd take on a more motherly role, making sure Danny is fed, clean, and safe.
Sam would probably be the first to be curious and excited. She might start researching about any possible connection between ghosts and dragons right away. Tucker would be a bit more cautious at first, but also amazed. He might be worried about how Danny can control this new form and if it will put him in more danger. Jazz would initially be shocked but then would try to be very supportive. She might offer to help Danny figure out the psychological aspects of having this new form and how it affects his identity as a half - ghost.
Sam would probably be very protective. She'd be the one making sure Danny was fed and clean. Tucker would be more into trying to figure out how to fix the situation using his tech skills. They'd both be really shocked at first, but then quickly adapt to taking care of Danny. They might have to come up with excuses for Danny's parents about why he wasn't around or why he was acting so strange.
Their friends might be really shocked at first. Tucker, for example, might not know how to handle the situation. He could be worried about how it will affect their group dynamic. Jazz might be more understanding and try to offer support, but also be a bit concerned about the practicalities of them being so young. Some of their other friends might be judgmental, thinking they're too young and not ready for this big step.
Danny would probably be extremely hurt. He might initially go into denial, not wanting to believe that his friends could do such a thing to him. He'd keep trying to reach out to them, hoping it was all a misunderstanding.
Well, one possible plot could be that Danny, while on an underwater mission to stop some ghostly threat near the ocean floor, gets hit by a strange spell or artifact that turns him into a mermaid. He then has to deal with his new form while also trying to keep his ghost - fighting duties going. Maybe he discovers new powers related to his mermaid form that help him in his battles.
At first, his friends would be in disbelief. But then, they would start to adapt. Dawn might see it as an opportunity to study a Pokémon from a different perspective. Gary could be both amused and intrigued, and might offer some scientific theories on how it happened. Overall, they would all be concerned for Ash's well - being and work together to either help him get back to normal or help him thrive in his new form.
I think his friends' reactions would vary. Naruto's closest friends like Shikamaru and Kiba would quickly realize that they need to support Naruto. They would see that despite the strange new form, it's still Naruto. Sakura would be more emotional, she might cry at first but then pull herself together to help. And Sasuke, he would probably see it as an opportunity for Naruto to gain more power, but also be cautious in case the Titan form has any negative effects on Naruto's mind.
His friends would be very shocked at first. Misty might be a bit worried and try to find a way to turn him back quickly. Brock, on the other hand, would be more curious and start observing Ash - as - Pikachu to see how his behavior has changed. His Pokémon friends would also be confused. Pikachu himself might be both excited to have a 'Pikachu - like' friend but also a bit sad that his trainer is in this strange state.
Danny would probably be heartbroken. He really cares about Sam, so finding out she cheated would be a huge blow. He might go into a state of denial at first, not wanting to believe it.