Raph usually gets really angry. He blames himself for not being there to protect Leo or he gets mad at the enemy who hurt Leo. He'll be the first to want revenge and will be very protective of Leo while he's injured.
Mikey is often the emotional support. He'll try to cheer Leo up by telling jokes or stories. He might also be a bit of a nervous wreck, constantly asking Donnie if Leo will be okay. Mikey will do little things like getting Leo his favorite food or making sure he's comfortable while he recovers.
Donnie, being the brains of the group, immediately focuses on finding a way to heal Leo. He'll use his scientific knowledge to analyze the injury, look for antidotes if it's a poison - related injury, or figure out how to treat any broken bones or wounds. He'll also keep a close eye on Leo's vital signs.
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 common scenario could be during a battle with a powerful enemy. For example, the Foot Clan might set up a complex trap and Leo, being the leader, might rush in to protect his brothers and get seriously injured. Another could be in a training accident. Maybe he's trying a new and difficult move and it goes wrong, resulting in an injury.
Raph would be the first to show his concern through anger. He'd be mad at himself for not protecting Leo better and also at the situation that led to Leo's injury. Mikey would be visibly distraught, constantly by Leo's side trying to cheer him up. Donnie would be all business, using his scientific knowledge to figure out the best way to treat Leo's wounds and help him recover.
Splinter, as their sensei and father figure, would offer wisdom and comfort. He might have some traditional methods to help with Donnie's pain and healing. And he would also use this as a teaching moment for all of his sons about the importance of caution and looking out for each other.
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.
Simon might be in a bit of a panic. Since he's her friend and also not as used to the dangerous situations as the other Shadowhunters. He'd try to do whatever he can to help, like getting help from more experienced Shadowhunters or trying to comfort Clary while she's in pain.
It could be an enemy. For example, a rival in a competition or a villain who has a grudge against Leo.
The other characters' reactions can vary. Pixal, if she's around, would use her technological knowledge to see if there are any internal damages to Jay, especially if it's related to his electrical components. The villagers of Ninjago might also show their support. They could bring food or offer shelter for Jay to recover. The reaction of the villains would depend on the situation. Some might see it as an opportunity to finish Jay off, while others might be impressed by his courage in getting hurt while fighting them.
Kagome usually gets very worried. She'll immediately try to use her spiritual powers to heal him if she can. If not, she'll search for herbs or other ways to help. Miroku would likely offer his support in a more practical way, like finding a safe place for Inuyasha to rest. Sango might be on guard, making sure that no more enemies take advantage of Inuyasha's weakened state.
The other characters' reactions can vary greatly. Some of the more powerful characters might see Atem's injury as a sign of weakness and try to take advantage. However, his true friends would rally around him. Bakura, in his good side, could offer some knowledge about ancient healing spells. Tristan would be the one to stand guard and make sure no further harm comes to Atem while they search for a solution. And the spirit of the Millennium Puzzle might also be actively involved in trying to heal Atem or guide the others on what to do.