I'm struggling to find a proper way to say how much I adore him. It's always wonderful to have characters you can see from a mile away that their muns love, and Arthur definitely falls into this category, and it shows in the amount of care and detail you put into his characterization and threads. He has such a great presence in the game and he's so layered, versatile, fun and easy to play with that anyone can approach him and build CR with him.
I've loved how you handled his development from the moment of his death up to now, his struggles with his old, wrong ideals and how he's opened up to and tried to learn more about magic. Same with the seemingly endless amount of things he's realized he didn't know or understand, of events he's had to rethink of now that he knows the truth about them, and how this all turns into a struggle to find himself and what his role in all of it was, what it will be in the future. I think you've handled it in such a fantastic way and you're doing perfect justice to his character.
I love the way he's bonded with others, as well. I love his awkward friendships, the ways he struggles with less formal interactions in which he can't simply assume the role of king and let that do most of the job. His threads with people like Jamie or Pan are amazing and I love being able to see that always trusting, gullible side of him come out even in a place like Kiseki.