She could be very brave. Growing up in Arthur's shadow, she might feel the need to prove herself just as worthy. So when faced with danger, she doesn't cower but stands up and fights, whether it's against a magical beast or a political enemy.
She could also be a key figure in the political intrigue of Camelot. As Arthur's sister, she has a certain status. There might be those who try to use her to get to Arthur or undermine his rule. So she has to navigate the dangerous world of court politics, using her wits and charm to outwit her enemies and protect Arthur and Camelot.
She could be really shy at first. Since Lucy is often quite outgoing in the series, her sister could be the opposite, taking time to warm up to people in the guild. For example, she might hide behind Lucy when meeting new members.
Another plotline could be a time - travel scenario. Draco gets sent back in time to Camelot. He becomes an apprentice or a follower of Arthur Pendragon. He then has to learn the ways of Arthur's court, deal with the politics and magic there, and in the process, he grows as a person, bringing back some of Arthur's wisdom to his own time when he eventually returns.
One possible reason could be a misunderstanding. Maybe Merlin did something that Arthur misinterpreted as betrayal or dishonesty. For example, if Merlin was secretly helping an enemy for a greater good that Arthur wasn't aware of at the time.
She could be shy and reserved, in contrast to Sakura's more assertive nature. This shyness might make her seem mysterious at first.
Saber from Fate/Stay Night would be an obvious choice. Since she is also related to the Arthurian legend, there could be a lot of interesting interactions between them. They could compare their experiences as king, or even fight side by side against a common enemy.
One common theme is gender role reversal. Since Arthur is typically male in the original legends, fanfictions with a female Arthur often explore how she navigates a world that has certain expectations for men. For example, she might face different challenges when it comes to leading knights or being accepted as a ruler.
The theme of redemption also pops up. If Arthur is placed in a darker or more morally ambiguous world, his quest for justice and honor can be seen as a form of redemption for the new world he's in, or it can be a test of his own values as he faces new challenges.
She might be very intelligent. Maybe she's the one who figures out the weaknesses of the various supernatural creatures they encounter. She spends her time in the library researching and comes up with strategies to keep everyone safe.
Friendship is another big theme. She could be making new friends in school or in the neighborhood, and fanfictions often detail the ups and downs of these relationships. For instance, she might have a falling out with a friend and then have to figure out how to make up.