If this is indeed a character from Stellar Theatre, than I'm driven to wonder if Hiro Suzuhira and Carnelian are good friends. I'd love to see a game that features both of these amazing talents at once.

Not really. It's one of those guest illustration collections where the publisher gets several artists to make art of the game, or whatever. They're relatively common.
Kalafina scanned the rest of the book here: http://moe.imouto.org/pool/show/779

I see, though I'm surprised you didn't upload a copy of Hashimoto Takashi's guest drawing here. Aren't he and Hiro Suzuhira actually good friends?

I'm didn't know that they are friends, but it wouldn't be surprising. I don't like to upload scans here where I wasn't the original uploader at Moe, but while I'm here: post #482496