Titans

Titans 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 & Behaviour
Titans greatly resemble human beings in that they are bipedal creatures with the same number and arrangement of limbs and fetures 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 no 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. Titans are compelled to seek out and devour humans for reasons 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.

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 forms of life (although they do sometimes kill horses in their attempts to get at their riders), and 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 in time to continue eating.

Titans' body structure and power source are very mysterious and apparently violate several known laws of science. 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 'Sonny' 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. However, most Titans are not very intelligent and many are little more than mindless beasts, easy to trick, distract or deceive. However, some Titans possess a vast level of intelligence comparable to or exceeding an experienced and cunning human-the Female Titan and to a lesser extent the Colossal Titan being primary examples.

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, including severed heads. It should be noted that the vulnerable area is where the 'real' human body would be for shape-shifting Titans, as proven by Eren Jaeger. After death, Titan corpses rapidly evaporate to skeletal remains, then nothing at all.

The Deviant Titan with hair covering its body is introduced as the first fully aware and intelligent Titan. It implies that it and its kind are far above humans, and he shows little regard for them other than taking interest in the 3D maneuver gear. It plans to bring the gear back somewhere to study, suggesting that there are other intelligent Titans.

Titan Types
Titans are classified by their size and behaviour. The various types seen so far in the series are:

Standard Titans:

 * 3-5 metre class: Titans that are roughly three metres in height, but less than 7 metres. These titans tend to be more human looking, as well as possess very large heads in proportion to their bodies. So far, none have displayed any real intelligence or unusual abilities. The captured Titan 'Sonny' was apparently a 3 metre class, with 'Bean' being a 5 metre class.
 * 7 metre class: this class has a greater tendency toards dysmorphic body proportions and they tend towards having a very large head as well as a semi-quadrupedal, ape-like stance. Arguably the most common Titan class.
 * 15 metre class: Was the largest class of common Titan until the haired titan's appearance, these tend towards more proportional bodies but also grossly enlarged mouths and a skinless appearance. They vary greatly in level of apparent musculature, some resembling out-of-shape old men while others possess extremely lean and athletic builds. The bearded titan, Annie Leonhardt's and Eren Jaeger's Titan forms are all of the 15 metre class.
 * 17 meter class: After the sudden appearance of the Haired Titan, this is now the largest of the common titan class. He is a deviant type, capable of speech and able to order the the lesser titans during the collapse of Wall Rose. It is for now his identity is unknown, but he could possibly be a shape shifter like Eren and Annie.

Unusual Titans:

 * Colossal Titan: the largest and most powerful Titan seen thus far in the series. At an estimated 60m tall, it is four times the height of any other Titan, and unlike most, apparently possesses a degree of intelligence and tactical planning to go with its strength. It has had a very limited appearance, but has played an important role in the story, as it is the only Titan powerful enough to break down the defensive walls surrounding humankind to allow other Titans in, something it has done twice already.
 * Armoured Titan:A Titan with advanced strength and agility, this Titan is muscularly well-built and has a hunched profile. While the Colossal Titan broke the part of the wall surrounding a town, this Titan purposefully broke down the gate leading into the interior of Wall Maria. Other than its initial apperance in Chapter 2, the Armoured Titan has not been seen in the story, however it is still feared within human settlements.
 * Female Titan: A Titan of very muscular and athletic appearance that lacks skin, the Female Titan is a Titan of unusual power and intellect, that is suspected of being a shape-shifter like Eren. First Titan shown that acts solely to kill humans and destroy military formations as quickly as possible, as well as maintain a running pace to keep up with horses over a long distance. She can also apparently command or control other, lesser Titans to some degree. She also has the ability to harden her skin, allowing her to protect her neck from attack and creating claws to scale the Wall with. In Chapter 31, it's revealed that Annie Leonhardt is the female type titan. After her capture, Scouting Legion scientist, Hanji Zoe, recovered a piece of her skin that had not disappeared.  Later analysis proved that the composition of this piece of skin and the Wall are identical
 * Eren Jaeger: A human boy and soldier, the main protagonist of the series. He has displayed an ability to shapeshift into a 15m Titan when he is a) injured and b) has a clear goal in mind. His real body is seen to be fused to the spine in the nape of the Titan body's neck when shapeshifted. The Titan body was initially under the control of his subconscious when performing actions, resulting in considerable unpredictability. However, with increased use, Eren has become more adept at using this form, but unlike the Haired Titan, cannot communicate verbally.
 * Deviant types: Any Titan that displays unusual behaviour or intelligence is classed as a 'deviant type'. For example, most Titans will mindlessly attack any human they see, but some will have the brains to charge to the centre of a military formation where they can do the most damage, rather than being easily distracted by other humans they encounter.
 * Wall Titan: During the capture of female type Annie, a portion of the Wall came down, exposing half the face of this 50 meter Titan. Although it looked at Mikasa, it appeared to be too weakened to move due to lack of sunlight. It is hinted that the entire Wall encases Titans such as this one. Armin hypothesized that the Wall takes advantage of the hardening ability of Titans to keep the Wall stable giving irony in that Titans are used to defend against Titans.
 * Haired Titan: A titan capable of speech with human intelligence who stands 17 meters tall or more than standard titans covered with hair like that of an animal who was able to order the titans during the collapse of Wall Rosa. He was able to figure out how soilders attack the titans' weakness on their necks and was able to deduce how the three dimensional gear allowed human soilders to jump. In chapter 35, it took Mike's 3D gear before he died for unknown purposes. It is possible that he is the first intelligent titan who isn't a shapeshifted human, assuming he isn't a shapeshifter himself.