It's hard to say for certain without more information, but the lack of the copyright message that appears on the other Mugen no Fantasia illustrations makes me strongly suspect this is actually Ren from Tsukihime. Does anyone know what book this is actually from?

Sonomi said:
It's hard to say for certain without more information, but the lack of the copyright message that appears on the other Mugen no Fantasia illustrations makes me strongly suspect this is actually Ren from Tsukihime.

• http://t-walker.jp/mugefan/illust/prd_list.cgi?odrid=30523&page=7&ilsid=00125
• http://t-walker.jp/mugefan/html/status.cgi?chrid=a21916

Thanks - I stand corrected.
The linked page gives her name as "Nekonekocat Iris Familiar" so I've tagged this as such.