Definitely not Ruka's mother, go back to ch.362, but she's definitely someone who knows her. I also don't think Nagi is in any the position to turn down a great tofu-protagonist idea.
Haha I really like the whole tofu protagonist and I think it might actually work since Maria can make a manga about Hop-chan (hope Hata can bring Hop-chan back somehow or maybe make a cameo on Nagi's manga XD)