Seth could be the one to help her. As her brother, he would be very concerned and might use his own knowledge of Fablehaven and its secrets to try and find a cure. He could also enlist the help of some of the friendly magical creatures they've befriended, like the fairies or the satyrs.
Grandpa Sorenson would definitely be involved. He has a lot of experience with the goings - on in Fablehaven. He might search through his library of magical books to find a remedy for Kendra's illness. He could also call on other allies in the Fablehaven community, such as the Knights of the Dawn, for assistance.
Well, it could be that her sickness is a result of some dark magic in Fablehaven. Maybe she was poisoned by a magical creature or a spell was cast on her. This would lead to her friends in Fablehaven having to find a cure, perhaps by going on a dangerous quest to find a rare herb or magical object that has the power to heal her.
Her best friend Veronica would probably be one of the first to help. Veronica is wealthy and could use her resources to get the best doctors for Betty.
Sango could also take care of Kagome. As a strong and kind - hearted woman, she would know how to nurse Kagome back to health. She might use her knowledge of herbs and remedies to make Kagome feel better. Plus, she would offer emotional support to Kagome during her illness.
Obviously, her best friends Sam and Freddie would visit her. They are always there for each other. Maybe Gibby would also come over with his unique brand of humor to try and cheer Carly up.
His teammates, Sasuke and Sakura, would surely be involved. Sasuke, although he may not show it as much, still cares about Naruto. He would keep an eye on Naruto's condition, and if there was any sign of danger, he would be ready to protect him. Sakura would be more hands - on with the medical care, checking his temperature, giving him medicine, and making sure he rests properly.
There could be many interesting fanfiction stories about Kendra in the Fablehaven world. One idea could be a story where Kendra discovers a new, hidden area in Fablehaven that has unique magical creatures. She might have to use her wits and the magical knowledge she has gained throughout her adventures to navigate this new area and make friends with the creatures there.
Anna would surely be the first to help Elsa. Anna has always been there for her sister, and when Elsa gets sick, she would do everything she could, like bringing her warm soups and staying by her side to keep her company.
His family members would likely help him. For example, his parents, Hank and Barb, have a lot of experience with superpowers and could try to figure out what's wrong.
In the fanfiction, it could be that Sailor Chibi Moon comes to take care of Sailor Moon. Since she looks up to Sailor Moon so much, she would do everything she can. She might even use her own little powers in a unique way to try and make Sailor Moon feel better, like creating a warm and cozy environment around her bed.
Padmé could also be involved. She is known for her compassion. She might use her resources to get the best medical help for Ahsoka. She could bring in the best doctors in the galaxy or some special remedies from Naboo.