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.
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.
Well, 'badass link fanfic' could be about a fan - created story centered around a character named Link who is depicted in a really cool, 'badass' way. Maybe it shows Link in some extraordinary adventures, with him being super powerful and fearless, fighting off hordes of enemies in a more intense and exciting way than in the original games.
One characteristic is that they involve a lot of passion. The lovers in badass love stories are deeply in love and their emotions are intense. For example, in the story of Cleopatra and Mark Antony, their love was full of passion that influenced political decisions and ultimately led to their downfall in some ways.
His attitude. Percy in fanfiction often has a 'don't mess with me' attitude. He doesn't shy away from a fight and is always ready to stand up for what he believes in. He'll face any opponent, no matter how powerful, without hesitation.
His unwavering determination is another factor. No matter how tough the situation is, Adrien in fanfictions doesn't give up. He is willing to risk everything to protect the people he loves and the city he lives in. This strong will and selfless attitude are what make him a truly badass character in many fanfictions.