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.
One common scenario could be during a battle with the villains. For example, when facing the Serpentine, Jay might get caught by their venomous attacks which could hurt him badly. Another situation could be an accident while training with his fellow ninjas. Maybe he misjudges a move and ends up crashing into something hard, like a rock or a wall in the training area.
Nya would be very worried. She'd probably stay by Jay's side as much as possible, trying to take care of him and find a way to make him better.
His friends, like Kai, would be very protective. Kai might get angry and rush into battle to avenge Lloyd's injury. Jay would probably be more worried and try to use his technical skills to find a solution to heal Lloyd.
Jay would probably be very vocal. He'd be shouting for help and freaking out a bit. Kai would be angry at whoever or whatever hurt Zane and be ready to fight back immediately. Lloyd, being the leader, would try to stay calm on the outside but be very worried inside and start coming up with a plan to help Zane.
Yuki would likely be very concerned. He has a soft spot for Tohru and would probably rush to her side, trying to find a way to help. He might even blame himself if he thinks he could have prevented it. Kyo, on the other hand, would initially be angry, but that's just his way of showing worry. He would be more hands - on in getting her the help she needs.
One possible plot could be that Jay tries to hide his sickness from his friends because he doesn't want to let them down during an important mission. But as he gets weaker, his performance suffers and his friends start to suspect something is wrong.
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.
Beast Boy would definitely be one of the first to react. He and Raven have a special bond in the Teen Titans, so if Raven gets hurt, he would be extremely worried. He might use his shape - shifting abilities to quickly get to her side or try to find help.
Macgyver would probably be very worried. He'd quickly assess the situation and try to stop the bleeding if she was bleeding. He'd use his resourcefulness to make a makeshift stretcher or bandage.
Li Shang would probably be very concerned. He might blame himself for not protecting her better. He would immediately call for the best medical help available and stay by her side until she recovers.