Titans are the primary antagonists in the Attack on Titan series. They are a mysterious race of humanoid giants that appeared roughly 100 years ago.Their origins, overall numbers, and their physiological workings remain a mystery to humanity at this point in the story.

Appearance

Titans greatly resemble human beings. 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 have a masculine body shape, although all of them lack sex organs.Titans almost universally possess widened mouths with an enormous number of small, square teeth, though some have been known to possess only canines. Still, the majority of Titans have been shown to lack canines, although possibly not incisors and use incisors 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.

Why do they eat humans?

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 its stomach-like organ with dead humans, it regurgitates them in a giant ball and continues eating more as soon as possible.

Behavior

Most Titans have no intelligence at all and 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.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. After the supposed breach of Wall Rose, Titans were seen moving in the dead of night when they should be inactive, traits they normally do not express.

Physiology

The 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. 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-fire way to kill them (known to date) 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 (although there are rare instances where 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. After death, Titan corpses rapidly evaporate to skeletal remains, then to nothing at all.How Titans react to pain stimuli varies from one to another. Many Titans will utterly ignore injuries which would be agonizingly painful to a human, such as losing limbs, being impaled through the heart, or having their eyes gouged out (even if such injuries will soon regenerate). In combat, some Titans will completely ignore armed humans who are attacking them, and simply proceed to attempt to eat whichever human is closest to them. Others seem vaguely aware of humans who are attacking them, and if injured will have a reaction that might be pain but more accurately seems like simple annoyance. Some actually do seem to react in pain and shout when they are injured.