Obviously, his brothers are very important. Donatello would be crucial as he could try to diagnose Leo's illness. Raph would be there to provide protection and strength if any threats come while Leo is vulnerable. Mikey would be the emotional support, trying to keep Leo's spirits up.
April and Casey could also be important. April might use her connections or knowledge to help find a cure. Casey could be on the lookout for any bad guys who might take advantage of Leo's situation. Splinter, of course, would be the guiding figure, offering wisdom and perhaps some traditional remedies.
His friends are very important. Rukia, for sure. She has a lot of knowledge about the spiritual world and could be the one to figure out what's wrong with Ichigo. Orihime with her healing powers also plays a big role. She might use her abilities to ease his pain or at least keep his condition from getting worse. Uryu could also be important as he might use his Quincy skills to analyze what's causing Ichigo's sickness.
His friends are the most likely ones. They know his habits and routines well enough to pull off some great pranks. For example, if Leo is always punctual, they could set his alarm clock wrong to make him late.
Obviously Naruto himself is the central character. His growth and new abilities would be the focus. Sasuke could also be important. He might have his own development and his relationship with Naruto could be explored further, whether they are friends or rivals again.
Naruto himself is obviously a central character. His growth and decisions will shape the story. Hinata could also be important as her relationship with Naruto might develop further. Shikamaru, being one of Naruto's smartest friends, could play a crucial role in planning for Konoha's future and strategizing against potential threats.
Hermione would definitely be an important character. She's smart and resourceful, so she'd be the one researching ways to restore Harry's memory. Ron would also be crucial as he's Harry's loyal friend. He'd be there to support Harry and keep him out of trouble. Dumbledore or other wise Hogwarts professors like McGonagall could also play significant roles. Dumbledore might have some secret knowledge or plan to help Harry, and McGonagall could provide guidance and protection at Hogwarts.
There might be a kind - hearted doctor in the 'baby bella gets sick fanfiction'. The doctor would examine Baby Bella carefully, give accurate diagnoses and prescribe the right medicine. Also, her best friend could be a character. Her friend might visit her often and bring her favorite toys or books to make her hospital stay or time of being sick at home more bearable.
Toriel is very likely to be involved. She has a motherly nature and would surely be concerned about Frisk's health. She would probably be at Frisk's bedside, offering comfort and warm food.
Sonic himself, of course. And his best friend Tails is very likely to be there. Tails is always by Sonic's side and would surely be there when Sonic is sick.
All Might is another character that is very likely to appear. As Deku's mentor, he has a big influence on him. He would be concerned about Deku's health in relation to his quirk. Recovery Girl, the school nurse, would also be an important character. She would be the one trying to diagnose and treat Deku's illness.
Harry Potter himself, of course. He has always been at the center of things in the wizarding world. His actions and decisions would likely have a great impact on any change that occurs.