i'm a bit angry how master!arthuria didn't uphold to fate tradition and give muramasa!shirou a raincoat, she's trying to go against nasu's irreplaceable fetish!
Given pretty much everything the artist draws is some form of Shirou/Artoria I'm pretty sure he's lying about his identity.
He'd draw LZO Shirou with Artoria before Muramasa was a thing, too.
That is Emiya's body, but Muramasa inside of him. From what I remember, Muramasa can't be normally summoned as a servant for various lore reasons, so he needs a host with ties to the grail, one with a personality that meshes well with him. I forget if he's also got a bit of Emiya's personality mixed in there as well or not.
Also it's been about half a year since since I acquired this information about him, so my memory is a bit hazy and I apologize if I'm wrong on any details here.