Titans

Titans (巨人) are the primary antagonists in the series. They are a mysterious race of humanoid giants that appeared roughly 100 years ago (107 years is the most specific number given during Eren's training) and rapidly exterminated humanity to the point of near extinction. Their origins, overall numbers, and how their physiology works remains a mystery at this point in the story.

Appearance
Titans greatly resemble human beings in that they are bipedal creatures with the same number and arrangement of limbs and features one would expect in human beings. They are all deformed by human standards to a greater or lesser degree, ranging from minor abnormalities in proportions (enlarged head, small limbs etc.) to apparently lacking skin and subcutaneous tissues, although they do not bleed from such deformities, indicating that they perhaps possess some other mechanism to compensate for this. The vast majority of Titans are masculine, although all Titans lack sex organs- their method of reproduction is currently unknown.

Titans almost universally possess widened mouths with an enormous number of small, square teeth. They apparently lack canines, although possibly not incisors, and use these in their primary activity – eating humans. They apparently do not possess a complete or functioning digestive tract, merely a stomach-like cavity that eventually fills up with what they swallow, forcing them to regurgitate after consuming a certain amount before being able to continue eating more humans they encounter in their rampage.

Behavior
Titans are compelled to seek out and devour humans for reasons currently unknown, as they do not derive any real sustenance from them; they did not taste human flesh for nearly a century after the walls went up, but this did not affect their activity or numbers. It should also be noted that once a Titan has filled what passes for its stomach with dead humans, it regurgitates them and continues eating more as soon as possible. They are not ordered to do so either, since without intellect they are unable to comprehend any sort of command.

It should be noted, as mentioned above, that Titans eat humans for no other apparent purpose than killing them. They ignore all other animals and any non-human forms of life. It appears to be a mere will to slay, perhaps triggered by a hormonal response of some sort. And almost all Titans act solely based on it.

Most Titans have no intelligence at all and many are simply mindless beasts, easy to trick, distract or deceive. However, some Titans, mostly Titan Shifters, possess a vast level of intelligence comparable to or exceeding an experienced and cunning human. For example, the Female Titan and to a lesser extent the Colossal Titan displays intelligence, a trait which is not found in an ordinary or deviant Titan.

The Ape Titan is introduced as the first fully aware and intelligent Titan and the only one to have any attribute related to cognition. It implies that it and its kind are placed far above humans, and it shows little regard for them other than taking interest in the 3D maneuver gear and their tactics and wondering why it and human beings share the same language. It plans to bring the gear back somewhere to study, suggesting that there are other intelligent Titans. It also implies Titans are divided into classes, with its class being the rarest of all.

The most dangerous aspect of the Titans is their unpredictability. They do not follow a strict set of guidelines and have seemed to increase in abilities and attributes as the series goes on. In chapter 38, Titans were seen moving in the dead of night when they should be inactive and breaching doors, skills they did not possess in the story. Although this could also be something caused by both how it is unknown how the Titans got past the wall, and why they were more or less led by an intelligent Titan capable of human speech.

Physiology
Titans' body structure and power source are very mysterious and they apparently violate several known laws of science, seemingly capable of spontaneously generating both energy and mass. Their extremely high level of activity and body temperature indicates the need for a massive intake of energy to fuel them, yet they apparently never need to eat at all. They do seem to become less active at night or if deprived of sunlight, and so it may be possible they draw their primary source of energy from the sun. Individual Titans seem to possess varying levels of stamina once deprived of sunlight-the captured Titan 'Sawney' became docile and near comatose after only an hour's deprivation, while fellow captive 'Bean' maintained a high level of activity three hours after last seeing sunlight.

Titans' bodies also seem unusually light for such large creatures - a severed arm is described as weighing almost nothing, and they are able to move at high speeds despite their size. Due to their size, Titans possess enormous strength and when combined with this lightness, they can put on amazing bursts of speed and agility.

In addition to their size, speed, strength and large numbers, Titans also possess incredible regenerative abilities - the only sure way to kill them is by slicing out the nape of the back of the neck. They regenerate lost and damaged body parts within seconds to minutes, Most of the time severed heads as well but it has been noted that there are rare times when the head will not regenerate. It should be noted that the vulnerable area is where the 'real' human body would be for shape-shifting Titans, as proven by Eren Yeager. After death, Titan corpses rapidly evaporate to skeletal remains, then to nothing at all.

Types of Titans
Titans are classified according to their size and abilities. The following are lists of various known Titans so far in the series.

True Nature
In the aftermath of the destruction of Ragako, the true nature of the Titans is discovered. The investigation performed by Hanji Zoe reveals that the villagers had been transformed into Titans by some unknown means, as a captured Titan is discovered to have once been Connie Springer's mother. Examination of test subjects show that no human body remains within the nape, but the spinal cord appears to operate as an organ separate from the main Titan body. As such, it is concluded that most Titans have completely absorbed the human within, leaving only the central nervous system intact as the Titans' weak point. The method used to slay Titans is revealed to sever the upper vertebrate of the human "pilot" (or what remains of them), effectively killing them via decapitation.

It is stated by Ymir that the Ape Titan is the one responsible for transforming the villagers of Ragako, though the method used remains a mystery. It is also unknown how Titan Shifters avoid being absorbed by their Titan form, though this is theorized by Erwin Smith to be the only difference between Eren Yeager and regular Titans.