Board Thread:Theories/@comment-17499145-20170509021147/@comment-14745711-20170527125646

While that's entierly possible, there is the fact that no one has said or done anything to imply that the 9th Titan is an unknown. The Marleyans haven't said anything to imply that the 9th Titan is missing. Also, the odds of Eren inheriting a Titan from the mainland AND being given two more are pretty dam slim. Admittedly we don't know there's no such thing as fate or a super natural force of sorts in this world so perhaps something could have directed this much power onto Eren, but I still feel its unlikely. Plus, besides that vision of the future (which only happened once) there has been nothing to imply Eren has a third Titan.

Really though, you're theory is pretty sound from what I can see. The main reason I think its wrong is because I don't think the author would do it that way. Isayama would have dropped a few more hints, even if they were utterly unintelligable at the time, if that were the case. I personally think that the 9th Titan is another Marleyan Warrior. Perhaps their most powerful one, but also perhaps not. I particularly like the idea that they might be a bodyguard for the Marley Royal family. That is, assuming they have one: they could be a republic or fascist dictatorship for all we know.