Shingeki no Kyojin/Attack on Titan/進撃の巨人

So those who know me, probably have heard about how I'm literally obsessed with this series at the moment. It is very well crafted and causes the reader to question every piece of information they're given. After a second read-through of the manga, I came up with some theories and realizations on several different topics in the series.

There will be a lot of spoilers, as I'm essentially analyzing Shingeki's plot.

First and foremost,

A lot of my speculation comes from the obvious answer that the people residing within the walls are NOT the last of mankind. We have a least four instances of this (Annie, Ymir, Reiner and Bertholdt). 

Ymir's Story vs Annie's Story

So, if you're caught up with the manga, you would have glimpsed pieces of both Ymir's and Annie's backstory. It seems that Annie was trained for infiltrating "mankind's territory", as is evident in her hand-to-hand combat training. She also seems to have a considerable amount of control over her titan form - which is very unique due to it's distinct female shape. I speculate that she is a human turned titan in a similar way to Eren.

With Ymir, however, it has been heavily implied that her story is very similar to that of Christa Renz/Historia Reiss and that she might be from a very powerful family outside of the wall. The evidence is in the backstory between Ymir and Historia (chapter 41). Ymir can also, like Annie, transform into a titan - but her form is not feminine in the least. It is, in fact, a "normal" titan in appearance. She has the ability to speak, however, leading me to believe she's a deviant class titan. (Refer to the deviant class titan section). Further evidence that she's like a human-turned-titan is in the small backstory flashback about her titan form, Reiner and Bertholdt.  Unlike Reiner and Bertholdt, Ymir seems to have no knowledge of the Ape Titan - this makes her a bit of a wild card. But both her and Reiner seem to be shooting for the protection of Historia -likely  making her an important figure to those outside the walls.

Reiner and Bertholdt

I am almost positive Reiner and Bertholdt are originally from the outside of the wall, as is evident when they ask Eren to return with them (and then kidnap him). They, along with Annie, were likely trained to infiltrate the walls of mankind. They are the Armored-Titan and Colossal Titan (respectively) and I speculate they were turned into titans in a way similar to Eren - which gives them their distinctive forms. I can only guess at their objectives, aside from that it is likely they were ordered to bring down the walls of mankind by a higher entity (which is likely the Ape Titan). 

Christa Renz

It is revealed in chapter 41 that her true name is Historia Reiss and she's the illegitimate daughter of a very important aristocratic family. According to the priest Hanji was talking with, she has the right to know all of the secrets of the wall and it is implied to that she has the right to other answers as well. Through her, we may learn just what happened 100 years ago and why there are titans in the wall.

Connie Springer's Village

I believe that all of the titans in the attack starting in chapter 35 are actually people from Connie's village. When all of the evidence is added together, it makes a lot of sense. There was no sign of a struggle with the titans, despite plenty of signs that they had destroyed the village. The stable remained full of horses. A titan unable to move was laying on Connie's house. This titan not only resembled his mother, but spoke the words "おかえり”, which means welcome home. In the first instance where Connie recalls what happened, Reiner dismisses it rather harshly. In the second instance, Ymir brushes it off with laughter and jokes in an effort to drop the subject. This indicates that it is likely possible for humans to be turned into titans.

Eren

Eren's ability to turn into a titan is likely due to the injection he received from his father shortly after the first wall fell. According to what memories he can recall, all of the answers to the current situation of mankind is in the basement of his old house - which was crushed by a giant rock. 

However, Eren's titan form seems to be the only one that only vaguely resembles him - with the exception being his eyes. 

The Walls

There are titans in the walls and it is very likely the walls are made with the hardened crystal skin of titans. In addition to the Ape Titan, this leads me to suspect that the "last of mankind' is not actually the last - but an experiment set up by titan "scientists". Furthermore, the author often used "birds in a cage" to describe Eren's opinion of the walls.

The Deviant-Class Titans vs Normal Titans

I am now reasonably certain that the reason titans differ is because most are humans-turned-titans that do not  have the ability to shift and deviant class titans might be ones that can return to their human forms. This has yet to be proven, but if one uses Ymir as a bit of a case study, she is definitely a deviant class in that she can talk and be reasonably well controlled when trying not to eat people. This seems fundamentally different from  Eren, Annie, Reiner and Bertholdt, who don't seem interested in eating humans at all.

The Ape Titan

The Ape Titan is very intelligent, but it's unlikely that he's an actually titan. It's more probably that he, and his colleagues, are crazy scientist-type creatures experimenting on the human race. This is evident in that he attempts to erase evidence that he was inside the wall (having Mike killed and throwing large rocks at Castle Utgard) and that he took Mike's 3-D maneuver gear saying it was interesting and he would take it back for analysis. 

There is speculation that the Ape Titan is Grisha Jager, Eren's father, but at this point, I'm going to say it's unlikely.