One common theme is the idea of sacrifice. The badass warrior often has to sacrifice something for the person they love. For example, in 'Akame ga Kill!', Tatsumi sacrifices his own safety and well - being for Akame and their comrades. Another theme is growth. The love story often forces the warrior to grow as a person. In 'Rurouni Kenshin', Kenshin grows and changes because of his relationship with Kaoru. He becomes more open and less isolated.
One great anime is 'Akame ga Kill!'. It features a group of badass warriors fighting against a corrupt empire. Tatsumi, one of the main characters, has a complex relationship with Akame, which includes elements of love. Their story is filled with action, sacrifice, and a touch of romance.
In these fics, Sakura often becomes badass through intense training. She realizes that she needs to be stronger to change things in Konoha or to prove herself. For example, she might train under a mysterious master or in isolation, like in 'Sakura's Solitary Training' where she spends months alone in the forest, honing her skills.
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.
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.
In some fanfictions, Snape has badass moments when he outsmarts Voldemort. For example, he manages to mislead Voldemort about the location of a crucial magical artifact. He uses his Occlumency skills to his advantage, hiding his true thoughts and intentions while presenting false information in a very clever way. This not only shows his intelligence but also his courage as he risks his life to deceive the Dark Lord.
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.
I'm not sure specifically as there could be many different 'bella the badass fanfiction' stories. It might be about a character named Bella who is portrayed as a badass, perhaps she has extraordinary fighting skills, a tough attitude, and gets involved in all sorts of exciting adventures in the world created by the fanfiction writer.