Definitely his family members. His mother, for example, would be a crucial supporting character. She would be worried sick about him and would do everything in her power to take care of him. His father might also play a role, perhaps using his godly powers in some way to ease Percy's suffering.
One possible plotline could be Percy's journey of dealing with the diagnosis. He might initially be in denial, then gradually accept it and start treatments. Another could be how his friends and family react, like his friends rallying around him to keep his spirits up.
Percy is surely a main character in this fanfiction. Depending on the universe it's set in, if it's related to a series like Percy Jackson, characters such as Annabeth, Grover, or his other demigod friends might also be important characters. They could either be the cause of Percy's anger or the ones who try to calm him down and help him through his angry state.
If Percy Jackson gets cancer in a fanfiction, it could completely change the nature of his adventures. His physical strength, which is crucial in the original stories, would be severely compromised. He might not be able to fight monsters as effectively. For example, he may struggle to hold his sword or run from danger. This could lead to new alliances, like relying more on his friends like Annabeth for physical protection while he uses his wits.
One possible plot could be Percy trying to hide his cancer from his friends at Camp Half - Blood at first. He doesn't want them to worry or think he's weak. But as the symptoms get worse, he has to tell them. Then they all band together to try and find a cure, whether it's through mortal medicine or some magical demigod - related remedy.
Grover could also be a main character. He might be at the scene when Percy gets drunk and try to use his nature - related powers to sober Percy up. Grover could also provide some comical relief with his reactions to Percy's drunken state, like his horrified expressions or his attempts to herd Percy like he would a wild animal.
Obviously, Percy would be the main character as the story is about him getting sick. But his friends like Annabeth and Grover would also be important characters. They would be the ones trying to help Percy get better, so they play a big part in the fanfiction.
The main characters are clearly Percy and Sally. Percy is likely a central figure who will have to deal with the consequences of getting Sally pregnant. Sally, on the other hand, is the one who is going to have the baby, so she will also be facing a lot of physical and emotional changes.
Obviously, Percy is one of the main characters. His friends like Annabeth might also be important as they could be the ones trying to find and rescue him.
One potential supporting character could be the Hogwarts professors. They may have different reactions to Harry's situation. Some might be more accepting and try to help, like Professor McGonagall who could use her Transfiguration skills to create things that are useful for Harry during his pregnancy. There could also be some characters from the Asgardian side, like Heimdall, who could be observing the situation from afar and may or may not choose to interfere depending on how he sees the future implications.
His best friend could be one of the main characters. They would likely be very concerned and play a big role in his recovery.