I will explain to you below to see why I USED TO put money on Rumi being Rum. You might find it useful somewhat, or not at all, and I totally understand and respect it. Please note, this is a theory I made a few months ago, so, you can consider it outdated with more given info. But, the guessing game is not over yet, and as a matter of fact, I know many now think Rumi is actually Rum, not gonna lie.
DISCLAIMER: Mostly for entertainment purposes. Don’t take it too seriously.
Reason 1: Psychological trap of Gosho: The common bluff
We have known pretty damn sure, that it is Gosho’s typical move to create new characters that look somewhat like Rum to divert us from thinking about who the real one really is. There have been so many characters in this Rum arc that could be Rum, and we have Kuroda, Wakita, Rumi, Iori, even Tsutomu, etc as proposed candidates. Creating the maze to put everyone in a guessing game is a very easy yet interesting trick to confuse us readers. So what is the truth? Who is Rum then? I think, that Gosho is bluffing and trying to make a diversion here and then reveals that Rum is someone obvious. Sounds stupid, but that is not what we have seen. Previously, we have seen many fans predicted that Subaru is Shuichi in fact, and Scar Akai was Bourbon, and the other way around. Either way, some turned out to be correct theorists, while some were not. Personally from my part, initially I thought Subaru was Bourbon, but then soon enough, I realized that he was Akai, due to similar physical traits, facial expression and (well, just a conjecture but I was right in the end). Conclusively, the fact that Subaru was Shuichi was very obvious to lots of us, so, it could be obvious that Rumi is Rum, though most of us are in denial as far as I have seen on forums. It is also quite lame to have Rumi become Rum by many people, because the name will garner attention from BO members and those who are involved in or investigating the case. However, things are not that simple and Rum can pull off this move for the reason 2 (see below). We might be bluffed into thinking of other characters being Rum, but I have my stance here.
Reason 2: Rum’s shenanigans in achieving his goal: The lure
It is rather clear, that since Rum is acting like a police officer investigating a case but just on the evil side, he needs to come up with some strategies to terminate the preys that were involved in the Kohji case. This is done by spying, constant observation from people around him, etc so on and so forth. Under the guise of an elementary school teacher, if Rum acts normally just like any other normal teachers we have seen like Kobayashi, etc then there is nothing to discuss here. What Rum is trying to do, is to act clumsily, look weird in others’ eyes and do the best he can to gauge reactions from others. Those who show reactions or some suspicious responses to him will be trapped by him because they have captured his attention. And so far, we have to admit that he has been acting very well – Conan has naturally shown his exceptional deductive skills in cracking cases right in front of him, in the guise of Rumi. For a 7-year-old kid to be that smart, there has to be something really special with this kid. Rumi has focused on Conan most of the time rather than Ai, the rat of BO. Again, I think Conan has been lured and now he is becoming a target, as a result. Why is he not focused on Ai right now? Well, BO thinks she is dead, that is one thing. Also, they have never thought of ATPX’s shrinking effects that’s another thing. So, Rum would be surprised when he sees that Shiho actually shrank into Ai right now. Though Ai looks like Shiho, Rum cannot just assume that and has to find ways to gauge reactions and info from this “girl” as well. This explains why Rumi, if turned out to be Rum, has not killed of Ai yet. Ai is surrounded by friends and others, so it is not easy to make that move at this time either. I sense that Conan is in great danger, but we shall see how things proceed forward.
On another spectrum, we have read that Gin has become more and more aware of the danger of Kogoro when he has accepted the offer to investigate the Kohji case. With many characters at present know of the brain behind Kogoro, naming Wakita and Kuroda as Rum suspects, it is very possible that Rum has learned of Kogoro’s motives and attempt to terminate Kogoro as well. I am not sure how well Rum is aware of the brain behind Kogoro, whether that exists or not, but once he finds out that there is actually a brain behind Kogoro, that brain needs to be eliminated as well for associating Kogoro with the Kohji case. By trying to look awkward, it is very easy for people to pay attention to her in some ways. We have seen Rumi’s grins multiple times already, suggesting that she has accomplished her goals. Whatever the goals are, one possibility is that now she knows something very important, either about Conan, or Kuroda as a matter of fact.
You can argue that Rum being in the guise of a teacher that is known to the public is not a smart move at all by the 2nd in command because he is putting himself into unnecessary troubles. But, with 3-layer disguises, he has some freedom of moves to play multiple roles at the same time, just like when he presents himself to different BO members through different forms. We know, that the Kohji case has been unsolved, and because it occurred in the US, Japanese police were uninvolved and are not aware of how the case could have happened with clouds of mysteries behind it. Therefore, by declaring the name with “Rum” in it, it is easy for him to actually observe who he needs to terminate. When things go out of hands, the alternate layers, etc will help him get out of all troubles and suspicions that could arise from others.
Reason 3: The 3-layer disguise setup
The reason why Ai has not been able to detect the black aura from Rumi, if she is Rum, can be attributed to the fact that Rum has completely changed his appearance by putting on 2 or 3 layers of disguises. It becomes really hard to track someone when at times the person acts in a certain way but different ways the other times. The sudden change from a fearful Rumi to a beast-mode Rumi really means something critical. Whether she is faking or not is to be determined, but we know that that is not the normal teacher. Beating up 3 thieves in a matter of seconds in the warehouse and knocking down the neighbor guy in one move was phenomenal. However, she tried to pretend that she was lucky in a way, and it was just something very normal, while in fact those are not normal things that a regular teacher was expected to do.
This disguise setup comes into play that allows Rum to create multiple possible guises and switch them over and over when convenient. We have not seen that ability thus far, but that creates a confusing profile of how Rum acts in certain situation: When no one observes him, he shows strength, and when there are people observing him, he tries to act fearful and runs away from the crowd. Also, don’t forget that below Rumi’s guise, it is a slim-looking Rum but is very physically strong and knows some martial arts to create the feminine look. Any guises of Rum are capable of doing all this, so it is extremely tough to really know what kinds of motives he is up to. The moment that Rumi was dumbfounded when confronted by Kuroda in file 988 is proposed by me that he temporarily forgot how to play his role in that situation when Kuroda suddenly became suspicious of her with an intense glare. I believe that Rum’s disguise abilities are taught by Vermouth, and it is not far-fetched to expect the 2nd in command to be multitalented, even in disguise.
Reason 4: The right eye
This is some new info that I pick up on from file 987. The reason why there are striking inconsistencies in describing Rum about which of his eyes is damaged, is because his eyes look the exact same, to the point it is impossible to really tell. We have seen Rumi does not look back to her right side often, only once, to Genta, when all the other times she looks back to her left side. It is quite probable that she has a dysfunctional right eye, but since the pupils can move and look so real, I think the right eye is the artificial eye, which matches with Rum’s eye.
Reason 5: The process of elimination
This comes from my own deduction on how each candidate cannot be Rum aside from Rumi.
Candidate #1: Kuroda
Kuroda was shown to be looking the Kohji case, meaning he is interested or curious about this case in some way. Suddenly pulling out the case and being focused intent on it means that he was involved in the case, but not that he read the newspaper to find out news about it. If that had been the news, then we can think that Kuroda is Rum, because he is following the happenings of the case, and how the cops are dealing with it, etc. Also, he looked at Rumi instantly when she was on newspaper. Based on this observation I deny that Kuroda is Rum. Also Kuroda had a permanently damaged eye, not an artificial eye like Rum.
Candidate #2: Wakita
This guy is known for his nomadic habits due to the jobs he has taken on, and also, his decent detective abilities. He has shown contempt when Kogoro failed to crack one case, but then showed utmost respect when sleeping Kogoro was activated and the case was solved. However, for a guy with that smart tendency, I expect him to learn that at least there is a brain behind sleeping Kogoro, based on the apparent contradictorily silly conclusions that Kogoro makes before and after he sleeps. According to MeitanteixX’s theory, the Pirates’ spirit that he refers to can also be linked to Rum, which the pirates drink. It is possible that he is in conflict with Rum,by saying that Rum is weak and that Anokata has put too much trust on him rather than Wakita himself (there is a quote of Wakita mentioning the Pirates’ spirit to Kogoro, much to Kogoro’s surprise). With the permanently damaged eye, it is not an artificial eye either, so I do not think Wakita is Rum.
Candidate #3: Iori
It is tough to refute the fact that this guy is Rum just yet, because he has only appeared in one case. His no ordinary stealth, excellent improvisation when confronted by Conan and Heiji, has the look of an assassin, especially like that of Shuichi a bit, and then removed his disguise later on to show the long hair, are all strong implications that he is the most solid candidate for Rum. However, my bet is that being a butler, it is quite similar to being a bodyguard, well not exactly the same things but there are some similarities, I think Iori is more likely to be Asaka rather than Rum, if anything. It is not known whether Asaka has managed to escape from Rum’s clutch in the Kohji case or not and if he is alive now, but, one thing we know for sure, that letting Asaka escape is a screwup of Rum, besides the dying message. Also, since I think that Kohji does not know of BO by the time, his dying message has little to do with Rum, more like about Rumi or something similar. After the Kyoto case, he hasn’t shown any more importance than he did before. It is also unknown as to whether he has an artificial eye, and because he has shown no sign of having a dysfunctional eye, I cannot assume he is Rum at this point.
Candidate #4: Chikara
This Shogi player needs more screentime, so for now I will consider him a filler character.
Candidate #5: Mary
Conan used to think Mary is Rum, but after Chapter 1005, it's certain that she's not. And she's one of our allies.
Of all these candidates, none is reliable enough to assume they are Rum to begin with, aside from Rumi, through this elimination method and my other observations from this character.