I LOVE THIS MANGA!
Okay,mystery geek mode on here so excuse me for sounding crazy XD
This reminds A LOT of And Then Were None by Agatha Christie,a classic mystery thriller. Anyone here ever read that? It's basically the same story.
10 people get stuck in an island. THEY ARE THE ONLY ONES THERE. As they get there,they see the following poem:
And then every single one of them is killed. EVERY SINGLE ONE. ONLY one of the 10 could have been the killer. AND YET, none of them COULD. Because they were all KILLED and there was no suicide,except by the last person,but that person wasn't the killer. Who is the killer then? Eh bien use your little gray cells to figure it out!(Cookie to who gets the reference)
Anyway, like I said they are both extremely similar.
Back to Doubt...I have 3 theories. No,4.
Theory #1: The killer is...Mitsuki!
WHAT? Okay,here is goes:
One: She is crazy about justice and rules,that has already been established.
That would(along with a sad past) justify her killings.
Two: She was supposed to be there by "accident" and yet they had her data.
That point was raised in the manga,but never solved.
If she was the killer then.....That is where my theory gets a bit frail. She had the motive,and what SEEMS like a clue. But WHY would she put herself in there? The only reason I can possibly think is that she (assuming that she is the killer) put herself in there was to frame Yuu. That sounds somewhat pushing it,since she had no way of knowing that she wouldn't frame herself with that, but considering that this is a manga not a book, she could have been a genius who planned every single detail, including the camera angles and how Yuu saw Eiji being killed, and didn't see Hajime. Calculating that Hajime was very smart and he wouldn't suspect Yuu anymore giving the circumstances, she
could[/b] be the killer.
Points against this theory:
Her birthday is the day after the whole game thing. Doesn't make any sense for her to do something like that before her birthday.
Theory #2: The And Then Were None Theory:
Some spoilers about the and then were none book here. Many actually. Major spoilers.
In the book,the way that 10 people were killed with one of them being the killer was actually very simple. One of them wasn't killed. He faked it.
The same thing could be happening here. Is Rei really dead? They didn't seem to check it. Sure it seems like she is dead, but is that even her?
The fact is,they never checked it. She was stabbed so they concluded she was dead, but a simple trick could have faked it.
If she is indeed alive, the only possible explanation is that she is the killer.
Points against it: She has a barcode. If they attempted to use it, they would find out that she is alive(probably).
Possible explanation for points against it: She thought that since they were just learning how to use the bar code system, they would think of using her barcode only as a last resort.
This explanation can be proven(or disproving) if they see rei's body one more time. If she is still there, then hell she is dead. If not,hell she is alive and the killer.
Theory #3: The Murder of Roger Ackroyd Theory:
Yuu is the killer. His reactions might seem to imply that he is not(specially given the fact that he is the main character,hence the Roger Ackroyd thing)
But he could be just acting. Killed Rei,then drugged himself.
Seems simple enough.
Theory 3 B) The Dr.Jerkyll and Mr.Hyde theory:
HE sent the message to Hajime telling him that the meeting had changed places. But he honestly didn't know that. If he had another personality, that would be easily explained. The other personality also would have made the research on Yuu and Mitsuki, and he could have even called her there(and Yuu doesn't know that) hence why they are all there.
Seems a bit pushy, but it sounds like it would be a plausible and awesome ending for Doubt.
Theory 4: The VERY,VERY Insane theory
The killer is....Mitsuki's dad.
Don't ask me why,I just suspect him. This isn't even a theory.
He isn't even a character.
....or maybe no one is actually dead and the whole thing is just a game and they don't know that?
Anyway, as for Hajime and Haruka,they don't seem like the killer to me.
Hajime seems to be way to....classic for a killer.
He is clearly the smartest one of the group, and the killer is usually smart.
And it is a cliché that the most obvious person is never the killer.
But again,if he is so smart why is he acting smart? He should know better!
Acting dumb would be the solution!
...so if he is acting smart,then he is the real killer?
....Theory number 5? haha
Haruka.....Humm that would be great for the "less obvious character is always the killer" cliché, but I don't have anything to say on her.
Sorry for the gigantic post, but I just am the biggest mystery lover ever....And I'm loving this manga!
so suspensful! it does seem a lot like saw, though. if the wolf turns out to be rei, eiji, or the bald guy it'll really end up being a total saw rip-off. at this point, i'm leaning toward either yuu or mitsuki just because they seem least likely to be the wolf, which in turn makes me most suspicious of them. does that make sense? 
