The pixiv account name in the URL has miya_katsura, and it sounds like a name, so I would go with that. No clear sign of what order that's in though.
I'm thinking katsura_miya is best as Miya appears here more as a given name and katsura as a family name. I might be assuming too much though, so I'm open to comments. I'm putting it through for now though, so I don't forget about it. I'll reverse it if that's the consensus.
