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.
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.
She could be shy and reserved, in contrast to Sakura's more assertive nature. This shyness might make her seem mysterious at first.
One great Arthur Pendragon hurt fanfiction is 'The Wounded King' which delves into his emotional turmoil after a major battle. It shows his struggle to regain his strength both physically and mentally.
Well, in such a fanfiction, they could team up against a common enemy. Maybe an ancient dark force that has been awakened in both their times. Their combined knowledge of magic from different eras would be a great asset.
A fanfiction that stands out is 'Arthur and Merlin: A Different Path'. This one takes a different turn from the original storyline. It imagines a world where Arthur discovers Merlin's magic much earlier. This changes the whole dynamic between them, and they face different challenges together. There is also 'The Adventures of Merlin and Arthur in the Enchanted Forest'. It's full of exciting quests and the development of their friendship.
Arthur Pendragon might fall in love with a noblewoman from a neighboring kingdom. Their kingdoms have a long - standing alliance. They meet at a grand ball held to strengthen the bond between the two lands. Arthur is immediately drawn to her grace and intelligence. They start writing letters to each other when she returns to her kingdom. Their love blossoms through these letters, but political intrigues and power - hungry lords try to break their relationship apart as they fear the united front of the two kingdoms through their love.