The Weasley family as a whole. They've always been like a second family to Harry. If Harry is standing up for himself against some form of wizarding injustice, the Weasleys would be there. Ginny, for example, would be very vocal in her support, and the older Weasley brothers might use their influence or their skills to help Harry in his stand.
Ron and Hermione are the most likely. They've always been by Harry's side in the original series. In the fanfiction, when Harry stands up for himself, they would surely be there. Ron might use his sense of humor to diffuse the situation if it gets too tense, while Hermione would back Harry up with her vast knowledge of wizarding laws and rights.
The Weasley family as a whole might also support Harry. They have always treated Harry like one of their own. Arthur Weasley, being an open - minded wizard, might understand Harry's perspective. Ginny, who is very close to Harry, would surely be on his side as well. They would see Harry as more than just a student defying a headmaster, but as someone fighting for what he believes is right.
One scenario could be in the Hogwarts corridors. Maybe some Slytherins are taunting Harry about being a half - blood. In the fanfiction, Harry decides he's had enough. He stands up straight, looks them in the eye, and firmly tells them that his blood status doesn't define him. He uses his skills in magic, like a well - executed non - verbal spell, to show he's not to be trifled with.
Hermione Granger would surely be one of the first to support him. She's always been by his side, helping him with research and providing intelligent advice. Ron Weasley would also be there, being Harry's loyal friend through thick and thin. Neville Longbottom could be another supporter. He's grown into a brave and capable wizard, and he would stand with Harry in his new role. Dumbledore, if he were still alive in the fanfiction, would likely offer his wisdom and guidance to Harry as he takes on this new and important role.
Ron would also be there. He might start out a bit reluctant, but then get into it. He could be Harry's partner in physical challenges, like having races on broomsticks or wrestling magical creatures in a controlled environment for fitness.
His friends, like Ron and Hermione, are very likely to support him. They've always been there for him in the original story, and in this fanfiction scenario, they would be even more important in his life. Hagrid could also be a great source of support. He has a big heart and has always cared about Harry.
Obviously, Harry Potter himself is the main character. He is the one who is teaching himself potions.
One great 'Harry Potter stands up for himself' fanfiction is 'The Power Within'. In this story, Harry decides he's had enough of being pushed around by the Ministry and starts to take matters into his own hands. He uses his knowledge of magic in unexpected ways to fight against the unjust rules. Another is 'Harry's Rebellion' where he rebels against Snape's unfair treatment in Potions class and shows his true skills and confidence.
I would recommend 'Harry's Solitary Stand'. In this fanfiction, Harry finds himself isolated from his friends due to a misunderstanding. Instead of waiting for them to come around, he stands up for himself and his actions. He embarks on a solo adventure where he discovers new aspects of his magic and his own strength. It's a really interesting read as it shows Harry's growth outside of his usual support network.
Hermione would be a very likely character to support Harry. She's his best friend and is always there for him in the original story. Ron too, as they are a trio. Neville could also step up, as he has his own growth arc and could understand Harry's feeling of being alone to some extent.
Obviously, Harry would be the main character. But there could also be other characters involved in the events leading up to him having to give himself stitches. For example, his friends like Ron and Hermione might have been there before and left him in a situation where he got injured.