I guess we can probably drop the spoiler boxes for now. Fujiwara pointed out that RUM, unlike all the other BO who have appeared so far, is written in uppercase English letters in all of the conversation bubbles. Other BO have their names written out phonetically in katakana. Fujiwara suggested RUM might be initials or an abbreviation for something, rather than a codename of an agent.
It was Akai who assumed RUM was meant to be a warning of an active agent. Kir might have meant something else.