Hermione would be a very likely character to help. She's smart and resourceful, and she would use her extensive knowledge of magic to come up with a plan for Harry's escape. Another could be Hagrid. He has a big heart and he's always been fond of Harry. He could provide a means of transportation, like his flying motorcycle or a secret passage he knows of. Also, Sirius Black might be involved. He's Harry's godfather and he would do anything to protect Harry from his family, so he could be a key figure in getting Harry out of that situation.
Definitely Hermione. She's the brains of the group and she would not stand by while Harry is in such a bad situation. She might use her skills in magic to create a diversion or a protective spell around Harry as he escapes. Then there's Ron. He's Harry's loyal friend and would be right there with Hermione, helping in any way he can, whether it's providing a distraction or helping Harry gather his things quickly. And Dumbledore could also play a role. He's always been aware of Harry's situation and might give Harry some crucial advice or a magical item that aids in his escape.
Obviously, Harry Potter himself would be the main character. He is the one who is escaping the wizarding world.
Hermione might also be a main character. She's Harry's clever friend and could be helping him plan the escape, using her vast knowledge of magic and spells. She might be the one coming up with the strategies to avoid detection and deal with any obstacles in their way.
Obviously, Harry is one of the main characters. His family members could include his parents, if they are alive in the fanfic's universe, or his siblings perhaps. There might also be his significant other, if he has one in the story.
In 'harry potter is betrayed fanfic', Luna Lovegood might be a character who betrays Harry. Maybe she is under the control of some mind - altering spell or has been deceived by a false prophecy that makes her think she is doing the right thing by betraying him. Also, Hagrid could be a shock betrayer if he is somehow forced to choose between Harry and his beloved magical creatures, and chooses the latter in a way that is seen as a betrayal to Harry.
Hermione could be a likely helper. She's smart and resourceful, and she would likely believe Harry if he told her his reasons for escaping Dumbledore. She could use her vast knowledge of magic and spells to create distractions or to find ways to bypass Dumbledore's security measures.
In this fanfic, a really cool plot could be that Harry has been secretly communicating with other wizards outside of his family through a magical object. They've been planning his escape for a long time. They send him a special charmed item, like a bracelet that can turn him invisible at will. He uses this to his advantage and slips away from his family during a chaotic moment, like when they're having a big argument. Then, he follows a series of clues left by his new friends to reach a safe haven, a hidden magical community where he can finally be free and learn more about his true potential.
Obviously, Harry Potter himself. He's the main character who is going to fight back.
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.
Obviously, the illithid would be a main character. It could be a mysterious and powerful being that Harry encounters. Also, Hermione and Ron might still be important characters. They could be either suspicious of the illithid or try to help Harry understand it better. Dumbledore might also play a role, perhaps guiding Harry in his interactions with this strange new creature.
Snape could also be a character in such a fanfic. His complex relationship with both Harry and Voldemort makes him an interesting addition. He might be in a difficult position, having to keep his cover as a double - agent while also trying to find a way to save Harry without revealing his true intentions to Voldemort.