I really can't help but suspect Iori Muga as RUM. Just a shot in the dark with the recent manga developments and RUM's eye for detail regarding the character usage (Hepburn/Hebon-shiki vs Kunrei-shiki romanization). Iori also seems to be into the Ogura Hyakunin Isshu (I'm guessing!) what with Momiji's engagements with karuta, so he might be keen on language use? It might be a long shot but it made sense in my head last night when I was mulling it over before heading to bed hehe. I'm also considering the possibility that the RUM codename may have been passed over, as well. My other suspect for RUM would be Kanenori.