Erak might be an unexpected character to help. Despite his rough exterior, he has a sense of honor. If he happened to be in the area when Will gets hurt, he could offer his ship as a means of quick transport to a place where Will can be treated. Also, the apprentices that Will has trained. They would look up to him and would be eager to repay his kindness. They could run errands, gather herbs for medicine, or keep watch while Will recovers.
Definitely his friends from Camp Half - Blood. Percy Jackson, for instance, would be likely to be involved. He might be the one to find Nico when he's hurt or be part of the group that's trying to rescue him. Annabeth could also be involved as she is often in the thick of things and would use her strategic mind to help Nico when he's injured. Hazel, being his sister, is very likely to be there. She would be extremely worried and do whatever she can to heal or protect him.
Well, Sasuke could be involved. He might be the one who finds Sakura when she's hurt and has to deal with his feelings for her while trying to get her help. Naruto is also a likely character. He could be devastated by Sakura's injury and vow to get revenge on whoever caused it.
Kakashi Sensei could also be involved. He is Naruto's teacher and mentor. If Naruto gets hurt during a mission that Kakashi is leading, Kakashi would surely be there. He might feel responsible for Naruto's injury and would do everything he can to protect him further and get him to safety. And then there's Pain. In some fanfics, Pain could be the one who attacks Naruto and causes him to get hurt, especially if the fanfic is centered around the events related to Pain's invasion.
Obviously, McGee is the central character as he's the one getting hurt. Gibbs would surely be involved. He'd be the one leading the investigation into McGee's injury, using his no - nonsense approach. DiNozzo would also be there, probably cracking some inappropriate jokes at first to lighten the mood but then getting serious as the situation unfolds.
Even some of the enemies of the Kanes could be involved in a different way. For instance, if Carter was hurt by a particular enemy, that enemy would be a central figure in the fanfiction. But there could also be unexpected allies among the enemies. Maybe a lower - ranked enemy realizes that Carter's death or continued injury could lead to a worse situation for them all, and they end up helping in some way, either by providing information about the cause of the injury or by secretly protecting Carter from their more evil comrades.
In some fanfictions, Jane Foster could play a role. If she's around Asgard at the time, being a scientist, she might have some knowledge or technology that could be used to treat Loki's wounds. Also, Valkyrie could be a helper. She's a fierce warrior and has a sense of honor. She might see Loki's injury as a chance to prove her worth or as a debt she needs to repay if Loki had previously helped her in some way. And of course, Frigga, if she were still alive in the fanfiction, would be a great source of comfort and healing magic for Loki.
Alex is very likely to be one of the first to help Kara. As her sister, she has a deep bond with Kara and would do anything to protect her. Alex has medical training and combat skills, so she can both treat Kara's injuries and defend her if needed. Another character would be Winn. He is a tech - genius and could use his knowledge to create something to help Kara heal faster or to find a way to counter whatever caused her injury. Also, J'onn J'onzz, with his shape - shifting and telepathic abilities, could be a great asset in protecting Kara while she's injured and in helping her recover mentally as well as physically.
Hermione would definitely be one of the first to help. She's extremely intelligent and well - read in magic, so she'd quickly start looking for spells or potions to heal Harry. She'd also stay by his side to comfort him and keep his spirits up.
Nobara Kugisaki and Megumi Fushiguro could also be among those who help. Nobara is a strong - willed and capable sorceress. She would be concerned about Nanami's well - being and use her skills to assist him. Megumi, being a calm and intelligent sorcerer, would likely come up with a plan to rescue or heal Nanami. They all share a sense of comradeship within the Jujutsu Kaisen world.
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.