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 shocked at first. After all, Merlin has been hiding his magic for so long. Maybe she feels a sense of betrayal initially, but then she might see an opportunity to have an ally in the world of magic.
The Duke Pendragon in the novel is often portrayed as a strong and fearless leader. He is usually skilled in combat, which helps him defend his land and people.
It's about a series of adventures. It usually involves a hero's journey through different worlds, facing various challenges and foes. It's full of action and mystery.
Pendragon Visual Novel is often about the Arthurian legends. It might include the adventures of King Arthur, his knights, and the magical elements associated with that era. It can have various storylines that explore themes like chivalry, love, and power struggles.
One possibility is that there would be a great deal of tension. Old Merlin has seen so much and has his own wisdom, while Morgana has her own powers and motives. Maybe Merlin would try to persuade Morgana to abandon her dark path. He could use his experience and the bond they once had to appeal to her better nature.
Their relationship could develop in a complex way. If Morgana ties up Merlin out of anger or suspicion initially, but then realizes she was wrong, it could lead to her feeling guilty. This might make her more open to Merlin's friendship in the future, and they could gradually build trust.
She might be shocked at first. Given that in the original story Morgana has her own complex relationship with magic, seeing Merlin's magic could stir a mix of emotions in her, like envy or a sense of relief that she's not alone in her magical abilities.
In a Jim x Morgana fanfiction, it could start with Morgana being trapped in a magical prison created by the Trollhunters long ago. Jim, out of his sense of justice and curiosity, decides to visit her. As he gets closer, he realizes that Morgana has a complex past and motives that are not entirely evil. Their interactions could then develop into a strange friendship that the other Trollhunters might not understand.