1st Choice (and, thus, my vote): Asaka (not Rum)
2nd Choice: Someone else related in some way to the case (or the victims) from 17 years ago
3rd Choice: A member of the NPA or the PSB or An agent of another intelligence agency
4th Choice: Another new BO agent or a BO goon
5th Choice: Vermouth or Rum or A red herring character who has nothing to do with anything
If Rumi turns out to be involved in the case from 17 years ago, it wouldn't be the first time Gosho kept trying to get us to think a teacher was a BO member, but, actually, was personally connected to an old crime committed by the BO.
Whether she was Amanda's bodyguard (Asaka), or is from an intelligence agency (NPA, PSB, etc.), I'd say it's still likely she's connected to the case from 17 years ago. Either way, I also think it's very likely that she was Koji's girlfriend.
I think she could only be a new BO member/goon if she doesn't know Shiho's/Sherry's face.
Rumi seems to be testing Shinichi/Conan—I see no reason why Vermouth would do that, considering how well she already knows him and knows of his capabilities.
For me, Rumi has a .001% chance of being Rum.
There's no such thing as a "a red herring character who has nothing to do with anything" in the mysteries of DC's arcs—such a character would serve no purpose. Especially when Gosho has red herring characters in the arc mysteries who go on to have purpose beyond just being a red herring.
“Life is infinitely stranger than anything which the mind of man could invent. We would not dare to conceive the things which are really mere commonplaces of existence. If we could fly out of that window hand in hand, hover over this great city, gently remove the roofs, and and peep in at the queer things which are going on, the strange coincidences, the plannings, the cross-purposes, the wonderful chains of events, working through generations, and leading to the most outre results, it would make all fiction with its conventionalities and foreseen conclusions most stale and unprofitable.”
“Education never ends... it is a series of lessons, with the greatest for the last.”
― Arthur Conan Doyle, The Complete Adventures of Sherlock Holmes and His Last Bow
"I have decided to stick to love... hate is too great a burden to bear."
— Martin Luther King Jr. (A Testament of Hope: The Essential Writings and Speeches of Martin Luther King, Jr)