There could be Hermione. Since she's always interested in learning new things, she might be involved in Harry's exploration of mysticism.
Perhaps some original characters created for the fanfiction could also be main characters. They could be other students who have a special interest in mysticism, or even some reclusive wizards who live on the outskirts of the wizarding world and are experts in this area. They could bring in new perspectives and challenges for Harry as he delves deeper into learning mysticism.
Snape is a character that could play a major role. Given his past with dark magic, he might be a reluctant mentor to Harry, guiding him in understanding the power and danger of it. Dumbledore, too, would likely be in the story, as he is always concerned about Harry's well - being and actions.
Besides Harry, there could be a character who is a descendant of an ancient elemental magic family. This character might initially be a rival to Harry in mastering the magic but later becomes an ally. Also, Dumbledore could play an important role. He might be secretly guiding Harry in his journey of learning elemental magic, using his own deep knowledge of the wizarding world's secrets.
One interesting aspect could be the new magical concepts introduced. For example, it might explore different forms of mysticism not seen in the original Harry Potter series, like ancient spells or unique divination methods.
There could be the mafia boss. He might be a tough and shrewd character who takes Harry under his wing at first but later becomes suspicious of Harry's actions. Another main character could be Harry's best friend in the mafia. They go through all the dangerous and exciting adventures together, and this friend might have different views on the mafia life compared to Harry.
There could be a doll collector named Alice. She might be Harry's friend or rival, always competing to get the best dolls in the story.
There might also be a love interest for Harry. This character could be someone who is also involved in the dangerous world of assassination in some way. She or he might be a fellow assassin or someone who is on the periphery of that world but still gets drawn into Harry's life, adding a romantic sub - plot to the story.
Hermione could be a main character. She might be the one who helps Harry understand and control his gorgon powers. For example, she could research in the library to find ancient spells or potions that can suppress the gorgon - like effects when they are unwanted.
Obviously, Harry is a main character. Hermione might also be there as she is good at potions and can help Harry. And Snape could be a prominent figure as he is the potions teacher.
Obviously, Harry is one of the main characters since it's about him learning parselmagic.
It could be the clan leader. He might play a crucial role in guiding Harry or challenging him within the clan.