So joking aside I think its supposed to be the fox on Eiji's Kitsune Udon shirt, as seen in the last page. Just, you know, outside the udon bowl (and a hideous monstrosity).
My lord that precious smile. It's like I can hear it. Oh fuck now I'm torn on who her voice actress should be if this ever gets an anime. Hanazawa Kana? Inori Minase? Ahh, Koga Aoi seems like a great choice too...