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.
Agent Madani is also somewhat underrated. Fanfiction could show more of her backstory and how she really feels about working in a world full of crime and violence. She could be given more complex relationships with other characters, not just Frank. For example, her interactions with other law enforcement agents or even some of the criminals she has to deal with.
Quicksilver could be one. In the movies, he didn't get as much screen time as he could have. In a multiverse fanfiction, he could be from a universe where his powers are even more enhanced and he plays a crucial role in a team - up. Another is Mockingbird. She has great combat skills and intelligence but isn't as well - known in the Avengers context. In a multiverse story, she could be a key figure in a different reality's Avengers.
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.
Huntress could be a great choice. She has her own sense of justice and skills that could add an interesting dynamic to the Knightfall story. Another one is Azrael. He has a complex backstory and his presence during Knightfall could be explored more deeply.
One underrated character is Black Widow. She has a wealth of spy skills and martial arts expertise. In Avengers Battleground fanfiction, she could be the one to gather intelligence on the enemies before the battle, and then use her combat skills to take out key targets during the fight. Another character is Quicksilver. His super - speed could be a game - changer on the Battleground. He could run circles around the enemies, delivering quick blows or stealing important items. And Bucky Barnes, the Winter Soldier, with his enhanced strength and combat training, could have some great storylines. Maybe he has to face off against some of his old enemies from his time as the Winter Soldier on the Battleground.
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.
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.