Hermione could also be an interesting choice in a different take. What if she was somehow influenced by a dark magic that made her seem like she was turning into a Slytherin in Gryffindor's 'clothing'? She would struggle with her new - found Slytherin - like behavior while still being in the Gryffindor house, and her friends would be very confused about the change in her.
Another option could be a new character. A transfer student, perhaps, who is really a Slytherin at heart but is placed in Gryffindor by mistake. This character then has to keep up the pretense of being a Gryffindor while dealing with their true Slytherin nature.
Perhaps there's a plot where a magical creature that has the appearance of a snake is disguised as a lion - like being. It could be used as a sort of spy within the Gryffindor common room. This would lead to all sorts of chaos as it tries to gather information for an unknown Slytherin - related plot.
The main character is Harry Potter. The lions are also key characters. There might be an old wise lion, like a Dumbledore figure among the lions. And there could be a young and adventurous lion cub that becomes Harry's best friend while he is growing up. There might also be some wizard characters who later interact with Harry when he discovers the wizarding world.
It could still be Harry Potter himself. He has a history of facing dangerous creatures and dark magic, so it would be natural for him to take on the challenge of the deadly snake.
Ron could also be the main character. He has his own courage and loyalty. He could be the one who has an unexpected connection with the poisonous snake, like being able to communicate with it in a special way that no one else can.
Obviously, Harry Potter would be one of the main characters. Then, the King of Lions himself could be a central character. He could be a majestic and wise being with his own backstory and motives. Hermione and Ron could also be important characters. Hermione might be intrigued by the new form of magic in the lion realm and Ron could be there for support and comic relief, as he often is in the original series.
Obviously, Harry Potter is one of the main characters. And the snake, which could be a unique creation within the fanfiction, is also a main character. It might have its own personality and abilities that are different from normal snakes in the Harry Potter universe.
In some fanfictions, Voldemort could also be a key character in relation to the snake king. Since Voldemort had a connection with snakes, he might either be in conflict with the snake king for power or try to control it. Additionally, there could be original characters created for the fanfiction. For example, a young wizard who has a special gift for understanding snakes and becomes the mediator between the snake king and the human wizards. This character could bring a fresh perspective to the story and help in building a relationship between the two very different worlds of humans and the snake king.
Obviously, Harry Potter would be one of the main characters. And the snake, which plays a crucial role as per the title. There might also be other characters from the original series like Hermione and Ron, who could interact with Harry in the context of his new relationship with the snake.
It could be Nagini. Even though in the original series she was more of a servant to Voldemort, in the fanfiction, she could have a different personality and form a bond with Harry.
It could also be a snake from the Forbidden Forest. There are many unknown and mysterious creatures in there. A snake from that forest might be more in tune with the natural magic of Hogwarts. Harry, being the curious and kind - hearted person he is, might come across it during one of his adventures in the forest and start a friendship with it.