In crestfallen Harry Potter fanfiction, some underrated characters that could be explored are Hannah Abbott. She has a connection to Hufflepuff house, and in a situation where things go wrong in the wizarding world, her loyalty and determination could be put to the test. Also, Blaise Zabini. He has an air of mystery around him, and in a crestfallen scenario, his family secrets could come to the surface and cause him distress. And then there's Parvati Patil. She has been more of a side character, but in this type of fanfiction, her experiences with magic and her relationships could be explored more deeply, especially if she is facing a situation that makes her feel crestfallen, like a rift in her friendship with Lavender or a setback in her magical studies.
Steve Randle is often overlooked. He could be explored more in fanfiction. He's a loyal friend to Sodapop and has his own dreams and fears. Fanfiction could show his life outside of just being a greaser, like his relationship with his family or his plans for the future.
Seamus Finnigan could be a great focus. He's often seen as just a side character, but in fanfiction, he could have his own moment. Maybe he discovers a new spell related to the Triwizard Tournament tasks that no one else knows about. He could use this spell to either help Harry or get himself into trouble while trying to prove himself.
Definitely Firenze. He made an appearance in this book, but in fanfiction, he could be more involved. Since he has unique insights as a centaur, he could play a key role in helping Harry understand the prophecies or the true nature of the events related to Sirius. Also, consider Madam Pomfrey. She's often overlooked, but she could be a great source of knowledge about past students and events at Hogwarts that might be relevant to the situation with Sirius.
Neville Longbottom is often underrated. In a fanfiction, he could have a more central role within the castle. Maybe he discovers a hidden herb garden in the castle grounds that has magical plants with the power to heal any wound. Another could be Luna Lovegood. She could find a secret room in the castle filled with strange and wonderful magical creatures that only she can communicate with.
One underrated character for Harry Potter OotP fanfiction is Seamus Finnigan. He had his own doubts about Harry in the original story. You could expand on that. What if he had a change of heart in a more in - depth way? Maybe he discovers something that makes him fully trust Harry again. Also, Parvati Patil could be a focus. She could have her own side story about her Divination skills and how they might play a role in the bigger battle against the Dark Lord.
Ser Harwin Strong is quite an underrated character. In fanfiction, you could explore his relationship with Rhaenyra more deeply. What were his true feelings? Another one is Mysaria. She has a lot of potential for exploration. Her role as a spy and her connection to Daemon could be delved into further. And there's also Laenor Velaryon. His story as a gay man in a society that may not fully accept him could be expanded in fanfiction.
Moaning Myrtle. She's central to the Chamber of Secrets story, but there's more that could be done with her. In fanfiction, she could have a more complex personality. Maybe she has some secrets of her own about the Chamber, or she could form an unexpected friendship with one of the main characters, which would change the dynamic of the story.
Colin Creevey could be a great character to focus on. He's often seen as just a bit of a fangirl - type character, but in fanfiction, he could have a hidden talent or connection to the Chamber. Maybe he has a family heirloom that gives him some special insight into the secrets of Hogwarts. Another is Myrtle Warren. There's so much more to her story than just being the ghost in the bathroom. She could play a more important role in the events of the Chamber, like having some knowledge that she didn't share in the original story.
Cho Chang. In a parallel universe, Cho could be a more central character. Maybe she discovers a new type of magic that is related to her Asian heritage. This could make her a target for both the good and bad sides. She could also have a more complex relationship with Harry or other characters, not just as a love interest, but as a powerful witch with her own motives and goals.
Luna Lovegood is often underrated. In a fanfiction focused on the Deathly Hallows, she could play a crucial role. Her unique perspective and knowledge of the magical world's oddities could help Harry and his friends in unexpected ways. For example, she might have insights into the Elder Wand's true nature that others overlook. Another underrated character is Kreacher. He could be the focus, as his loyalty to the Black family and his knowledge of the old magic could be key in the fight against Voldemort, especially when it comes to the secrets related to the Deathly Hallows.