It occurs to me while I'm writing my fics that I've probably taken more than a few creative licenses on how I interpret stuff that may or may not happen/be supported in the series. So I'm putting my notes down, to sort of remind myself of my characterizations of the characters, I suppose. I make no guarantee that any of these would even be accurate, however. After all, I have no way to truly know what the series writers had intended.
Mostly disregarding things that are outright stated or shown in the series, here are my thoughts as to who could most likely have tumbled onto the truth...who may have suspected a little...and who have no clue whatsoever.
I think Teppei knows very well, or very much suspects Mirai of being Mebius almost right from the start. To me, episode 3 makes this point pretty clear. Another indication for me is when Teppei says things like 'I think he (Mirai) is serious' about whatever common thing everyone should be familiar with. And in 47, he's the only person who figures out who Mirai is.
Teppei is a geek, true, but he seems very observant. I get the impression that one can't seem to get much past him. That said, although Teppei is someone (I'm inclined to say almost the only person in GUYS) who can see the big picture, I have to admit that he mostly does not seem to display much common sense when dealing with events and things where he himself is personally involved. (See episodes 14 and 33, for examples.) However, he can take comfort in the fact that such failing is in no way unique to him.
Anyway, back to the subject at hand, I'll definitely place Teppei firmly in a 'have a clue' camp. I'm probably biased, but Teppei is a clever character. Logically, Mebius' identity is not something that should have eluded him for long, if it ever had at all.
I'm not quite sure about Konomi as much as I am about Teppei. Still, she doesn't seem to be so oblivious, or at least she has never really been shown to be entirely without a clue.
Can't really point to specific examples to back up my thoughts, but in general, Konomi usually seems to be able to catch nuances in relationships. Her being a nursery school teacher may help with that. She appears to think of Mirai as a kid--specifically, as one of her charges--and doesn't seem at all bothered by that thought. One might say that she is simply weird, but I rather think she intuitively realizes that there is something more to Mirai other than his childlike wonder.
In conclusion, while Konomi may or may not suspect Mirai of being Mebius, I don't think she would be entirely surprised to find that it is true. In any case, I'm leaning toward the opinion that she probably does have some idea subconsciously in the back of her mind. (For some reason, I'm reminded of 38, even though that episode really has nothing to do with her knowing or not knowing about Mebius.)
In my opinion, George is a somewhat inconsistent character. In some matter (episode 38), he appears pretty quick on the uptake , and yet in some other matter (episode 30), he seems as clueless as most of the others.
George is another person who seems to think of Mirai as a kid (though probably not as a pre-school kid), and, apart from his initial resistance, sort of takes Mirai under his wing. There are instances of such in episodes 29 and 45, though there are probably more.
Anyway, George could probably go to either a 'have a clue' camp or a 'have no clue' camp. And annoys members of either camp so much that they long to kick him to the other camp. (Okay, I really like the meddler aspect of his personality, the one where he insists on going against the norm, even though I don't really remember seeing much of it outside of episode 14.) Seriously though, the truth--that Mirai is Mebius--probably set George back at least a little bit, even though I think he would most likely be pleased by the revelation. He probably hasn't dreamed of being a sort of mentor to his childhood hero.
Though it doesn't seem to speak well of his common sense, I honestly believe it hasn't occurred to Ryuu at all to suspect Mirai of being Mebius. Unlike most of the others, he seems straight-forward, so I can't really imagine him hiding that he knows about Mirai, if he actually does. (He would probably confront Mirai right away, even if he has only vague suspicions.)
One such indication to me is Ryuu seems genuinely perplexed (or, more accurately, goes 'WTF') whenever Mirai doesn't seem to understand whatever common thing everyone should be familiar with. To be fair, his reaction is probably part of his 'act-exactly-as-he-feels' personality. He seems incapable of truly deceiving others, which is a good trait, I think, but it makes him easily a victim of betrayal. And, understandably, he is upset when such betrayal occurs. But I think Ryuu is also quick to forgive, so viewers are not really subjected to his endless angst.
I seem to get off the point, but I'm pretty sure Ryuu has no clue about Mirai and is likely to remain that way if someone does not tell him otherwise.
In some ways, I think Marina is very similar to Ryuu--and most likely the reason they butt heads so easily: they both speak their (often blunt) opinions loud and clear and don't much care if they offend anyone with said opinions. Also, they do match each other perfectly in the clueless department in episode 38. However, Marina seems to have at least a little advantage over Ryuu in detecting something intuitively. After all, she is the person who has her teammates pegged by their stereotypes, and got all of them right.
(I'm a little disappointed that we'll never know which sobriquet(s) others use to refer to Marina. Or maybe they are so fearful of her they never come up with one? OTOH, if I'm not mistaken, I think Ryuu actually does have one not too flattering name for Marina, though I'm blanking on exactly what it is...)
Sadly, I can't seem to recall the time Marina has any meaningful interaction with Mirai (except perhaps for 23 and even that episode is not about him; it could have been anyone else in place of Mirai). But at any rate, in the matter of Mebius' identity, Marina probably have less clue than Konomi, but more clue than Ryuu, I think.
Thing is, it is true that I cheat and write this post after I'd seen all fifty episodes. But honestly, it wouldn't have significantly altered my opinions had I still been on the episodes in mid-twenties. The characterizations/character developments in Mebius are just that good.
(And I still have no idea why, as I go down this list, it seems less about Mirai's secret and more about each GUYS member's personality.)
Just for the sake of completion...the rest of GUYS:
Sakomizu and Misaki: The series makes their stances on this subject clear, so I'm content to leave it at that. Although I'll just say that I agree, since Sakomizu and Misaki are not shown to do anything contrary to that fact.
Toriyama and Maru: I think these two wouldn't get a clue even if it comes up to bite them. Unless that happens in episode 16, probably; for some reason, Toriyama and Maru just seem so unlike their obnoxious selves in 16. Or in 36, but what happens in that episode really says nothing of their intelligence or lack thereof.
One shot and a few shot characters, even those that are part of GUYS, are not included here, mostly because