User blog comment:G4bbster/snk discussion // spoilers/@comment-10733658-20130801082238

I think at this point, we can state with almost complete certainty that Connie's village was the source of the strange Titans that appeared within Wall Rose. They haven't explicitly stated it, but all evidence points towards that conclusion.

Hard to say at this point. I have a strong feeling that Reiner, Bertolt, and Annie may not even have all the information themselves. Regardless of whether they were like Ymir and spent time trapped in Titan form, we do know that they started their mission while still fairly young. Reiner admits that they didn't completely understand things back then, and Ymir refers to them as "small fry" while implying that killing them isn't going to accomplish anything. And considering the themes in the series of people being used/expendable, and not given all the information before throwing their lives away.....yeah. I think there's bigger things in play that even they don't know about.

Grisha is a big mystery. We don't know anything about his motives, or what he's been up to. We know he left just before the attack on Wall Maria to see someone in Wall Sina, and told Eren to go to the basement. But other than that......he's a huge question mark.

Again, impossible to know at this point! He seems to have been dormant or otherwise kept away from the modern world for some time, since he doesn't even know what language the humans speak or what the 3DMG is. Ymir stated his actions this time were to probably "test" the strength of humanity. And clearly, whoever the Titan trio are working for think he's important to their goals.