Some of the Weasley family members could support Harry. Mrs. Weasley, with her motherly instincts, might take Harry under her wing. Ginny, as Harry's love interest in the original series, would also be there for him. And Fred and George could offer their unique brand of humor and mischief to cheer Harry up during his difficult time.
Definitely Hagrid. Hagrid has a big heart and he has always been fond of Harry. He could offer Harry a place to stay at his hut when things get tough at Hogwarts. Also, Sirius Black (if he's still alive in the fanfiction) would be a great support. He's Harry's godfather and would likely fight for Harry even if his parents disowned him.
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.
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.
Ron might support Harry at first. Since he's Harry's best friend, he would be on Harry's side without really knowing the full story. He'd probably offer to help Harry with some of his revenge plans, like distracting Ginny while Harry sets up a magical prank.
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 from Hogwarts like Ron and Hermione. They are always by his side and would probably be more than happy to help Harry get revenge on the Dursleys. They might use their magic skills together to create some really interesting revenge scenarios.
Definitely Hermione. She would be the one to encourage Harry to keep going and not let his parents' disowning define him. Neville could also be a great support. He has his own experiences of being underestimated, and he could share his strength with Harry. And Luna. Her unique perspective on life and magic could give Harry new ways to think about his situation, and she would offer him her unwavering friendship.
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.
Neville might be an unexpected supporter. He has grown a lot in courage and magical ability. He could offer his unique perspective on plants and Herbology - related magic. In a situation where Dumbledore least expects it, Neville's knowledge of magical plants could be the key factor. For example, Neville could use a plant's special properties to disrupt Dumbledore's magic or create a distraction that Harry can take advantage of.
Obviously Harry is one of the main characters since his name is in the title. It's also possible that there could be characters related to Ravenclaw, like Luna Lovegood who could be involved in the story about Ravenclaw money. Maybe there are also some original characters created by the fanfic author to help drive the plot forward regarding this money issue.
Some of the older students like the Gryffindor prefects could support Harry. They understand the importance of a just Hogwarts environment. Neville Longbottom could also be a strong supporter. He has grown a lot in courage throughout the series, and this would be his chance to show his loyalty to Harry and fight against Umbridge's unfair treatment of the students.
Well, Harry is obviously the main character since it's about him disowning himself. But his friends like Ron and Hermione are also likely to be important characters as they are closely related to Harry in the original series and would be affected by his decision.