gon gets hurt badly

It could be an enemy. For example, a rival in a competition or a villain who has a grudge against Leo.

Maybe Leo is in a fight and gets seriously injured. His friends then have to find a way to save him.

When Naruto is badly hurt, it might lead to a shift in power dynamics in the village. Some characters who were previously in the background might come to the forefront to fill Naruto's shoes temporarily. This could create new alliances and rivalries within the village, which would drive the plot forward in interesting ways.

One reason could be a powerful new enemy. Maybe an enemy with a unique jutsu that catches Naruto off - guard. The consequence would be that his friends would be extremely worried. They would probably rush to his side to protect him and find a way to heal him. His enemies might also take advantage of his weakened state to launch more attacks on the village.

One common way is in a fight with a powerful enemy. Maybe a new mutant or a really tough Foot Clan member. Another way could be through some sort of accident, like a trap that was set for the turtles.

There's also the possibility that it's not a dark force at all. It could be a well - meaning but misguided wizard who accidentally causes Harry to be badly hurt. For instance, during a training session that goes wrong or while trying to perform a difficult spell to protect Harry but ends up harming him instead.

In the fanfiction, it might be that a mysterious figure is responsible. This person could have a vendetta against Amy's family or the stables she works at. They could set up a trap, like loosening a fence rail that Amy's horse jumps over, causing the horse to stumble and Amy to be thrown and badly injured.

Leo would probably take charge immediately. He'd be focused on getting Mikey to safety and finding a way to treat his injuries. He might be blaming himself a bit for not being there to protect Mikey better.

One possible reason for Natsu to return badly hurt in a fanfiction could be a very tough battle. Maybe he faced an extremely powerful enemy that was far beyond his initial expectations. For example, if he encountered a new type of dark mage with some unknown and dangerous magic. This could lead to him getting seriously injured and having to struggle to return.

His friends like Joey and Téa would be extremely worried. Joey would probably be the first to jump into action, ready to fight whoever or whatever caused Yugi's injury. He might shout something like 'No one messes with my pal Yugi!'. Téa, on the other hand, would be more focused on Yugi's well - being, trying to comfort him and call for help.