There could be his arithmancy teacher as a main character too. This teacher might be strict but also very knowledgeable, guiding Harry through the difficult subject.
One interesting plotline could be Harry's initial struggle with arithmancy concepts. He might find it difficult to understand the complex numerical magic at first, but as the story progresses, he starts to see how it relates to his overall magical abilities.
In the 'Harry Learns Alchemy' fanfic, Harry is the central figure. He is on a journey of learning alchemy. Hermione would be a key character as she is very studious and would be eager to help Harry understand the complex concepts of alchemy. There could be a mysterious old wizard who acts as a mentor to Harry in his alchemical studies. And Ron, as Harry's best friend, would be around, maybe not fully understanding alchemy but being supportive in his own way.
Obviously, Harry Potter is one of the main characters as it's about him learning magic. He is the central figure around which the story revolves.
Obviously, Harry would be one of the main characters as it's about him learning lost arts.
Obviously, Harry would be one of the main characters. Since it's about him learning family magic.
Since it's about Harry learning the old ways, Harry Potter is surely one of the main characters. There might also be some old - time wizards or witches who teach him the old ways.
Obviously, Harry would be one of the main characters. Since it's about him learning about the Force. There might also be some Star Wars characters if it's a crossover. Maybe a Jedi like Obi - Wan Kenobi could be there to teach Harry about the Force, or a Sith to show him the dark side.
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.
One possible fanfic could be where Harry discovers a hidden connection between Runes and Arithmancy that helps him in his battle against Voldemort. Maybe he finds ancient runes that, when decoded using arithmancy principles, reveal a powerful spell or a secret weapon.