There might be a human female warrior who has hiccups that are both a hindrance and a unique part of her identity. In some fanfictions, she could be a sidekick to a more well - known hero, but her hiccups end up saving the day in unexpected ways, making her a popular character.
One characteristic is their independence. For example, in 'The Handmaid's Tale', Offred, though in a very restricted and oppressive society, still tries to hold on to her sense of self and make small acts of resistance, showing her inner independence.
Badass female characters in romance novels are often independent. They don't rely on others to solve their problems. For example, in 'A Court of Mist and Fury', Feyre is able to make her own decisions and stand up for herself. They are also brave, like Jude in 'The Cruel Prince' who isn't afraid to face the dangerous faeries. Another characteristic is that they have their own goals. In 'Red Queen', Mare wants to change the world for the Reds, and she doesn't let her romantic feelings completely overshadow this goal.
One great book with badass fictional characters is 'The Lord of the Rings' by J.R.R. Tolkien. Characters like Aragorn are badass. He's a skilled warrior, a natural leader, and has the courage to face the dark forces of Sauron. Another is 'Harry Potter' series by J.K. Rowling. Harry himself is a badass as he battles the powerful and evil Voldemort throughout his life at Hogwarts and beyond, not to mention his friends Hermione and Ron who are also very brave. Then there's 'The Hunger Games' by Suzanne Collins. Katniss Everdeen is extremely badass. She volunteers for the Hunger Games to save her sister and shows remarkable survival skills and defiance against the Capitol.
You should check out 'The Dark Tower' series by Stephen King. Roland is a badass gunslinger on a quest to reach the Dark Tower. He faces numerous challenges and enemies along the way, and his skills with his guns and his unwavering determination are really impressive. 'Ender's Game' by Orson Scott Card also has a badass character in Ender. He's a child prodigy who is trained to fight against an alien race. He has to endure a lot of psychological pressure and still manages to outwit his enemies.
Well, it could be characters like Draco Malfoy. In the original series, he had his moments of growth and in fanfics, he might be one of the main Slytherins to defend Hogwarts. Maybe he finally realizes the importance of the school and his own values change. He could be joined by other Slytherins who were perhaps overlooked in the official story but show their true courage in the fanfic.
Obviously, Bella is one of the main characters as it's named after her. There might also be other characters like her friends who support her in her badass adventures, or her enemies who she has to face off against. It could also include love interests, depending on the nature of the story.
Issei is the central figure in 'badass issei fanfic'. But there are other characters too. For example, the villains he faces. They are designed to test his new - found badass skills. And then there are his comrades from his group who interact with him in different ways, depending on how his badass personality affects the group dynamics.
Obviously, Bella is one of the main characters since it's about her being a badass. There might also be other characters from the original story she is from, like Edward or Jacob, who could be involved in the new storylines around badass Bella.