Titans (Attack on Titan)

Titans (also known as Kyojin) are are a race of humanoid giants that serve as the main antagonists of the anime/manga franchise Attack on Titan. Throughout the series, these towering monsters have been terrorizing humanity, slaughtering and devouring humans en masse and have driven mankind to the brink of extinction. The remnants of humanity have been forced to take refuge in the city of Shiganshina, erecting massive walls around it to keep the Titans out. Although the Titans have been present since 100 years before the events of the series began, very little is known about them. Their origins, their physiology and the reasons behind their consumption of humans remain shrouded in mystery.

Appearance
Titans greatly resemble nude 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 reproductive organs (they have no need to reproduce since they are humans transformed through injection of Titan serum. The exact composition of Titan serum is unknown, but is said to be Titan spinal fluid.).

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.

Behaviour
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.

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. So it is possible that the Titans are trying to (by chance) eat a Titan Shifter, thereby becoming a Titan Shifter and be able to resume human form. As is shown in Ragako, humans are transformed into Titans. When Eren Yeager is injected with something by his father, it is safe to assume it is a "Titan serum" because one must eat a Titan Shifter as a Titan to become a Titan Shifter. It is shown that Titans try to eat Titan Shifters in Titan form.

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. For example, the Female Titan and, to a lesser extent, the Colossal Titan display acute intelligence, a trait not found in an ordinary Titan. A Titan encountered by Ilse Langnar possessed enough intelligence to speak and bow before her, calling her "Lady Ymir".

However, although most Titans show no visible or audible form of communication, they are apparently aware of - and able to deduce hostile intent from - other Titans and Titan Shifters, as demonstrated when Eren Yeager first transformed into his Titan form and was attacked almost immediately by other Titans who perceived him as a threat. Titans do possess the ability to roar, but don't appear to possess the vocal organs necessary to speak human languages.

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. It has been theorized by Hange Zoe that since Titans receive sustenance from sunlight, moonlight would also be sufficient enough, due to the moon being reflected sunlight.

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 possess vocal organs and are able to emit simple sounds like roars and screams, yet they apparently do not need to breathe. 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. Hange has theorized that new Titans may be able to use the small amount of sunlight reflected off of the moon as energy at night time. She reasons that this is why there were so many Titans storming Castle Utgard during the supposed breach of Wall Rose, as it is believed that many of those Titans had been freshly created from Ragako.

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 if the nape is cut through or blown off). 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. When struck at this site, the human controlling the Titan (or rather what remains of them) is decapitated, killing both. 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. In Hange's experiments, Bean shrieked in pain when they gouged out his eyes with a heated metal spear, but Sonny seemed oblivious to the fact that Hange had impaled him through the heart region using a spear.

Types
Titans are classified according to their size and abilities.

Common Titans

 * 3-6 Meter Class - Roughly three meters to six meters in height, these Titans tend to be more human-looking, possessing large heads in proportion to their bodies. So far, none of this type have displayed any intelligence or unusual abilities.
 * 7 Meter Class - These types of Titans have very large heads with a semi-quadrupedal, ape-like stance. Arguably the most common Titan class.
 * 8-14 Meter Class - These Titans resemble humans the most. They seem to be more energetic and physically fit than other Titans.
 * 15 Meter Class - Among the largest of the Titans, these Titans have more proportional bodies but with also grossly enlarged mouths.

Uncommon Titans

 * Abnormals - Also called Deviant or Aberrant.  Unlike most Titans which mindlessly attack and devour humans on sight, Deviant types often ignore nearby humans and charge to more important locations where they can do more damage. Some Deviant types possess noticeable physical deformations, such as lacking skin in certain areas or carrying less weight than others.
 * Wall Titans - First discovered inside Wall Sina when the Female Titan tries to escape over it. These type of Titans have a similar appearance to the Colossal Titan and can be found inside the walls. Although they appear to be alive, they are weakened and immobile due to lack of sunlight. A Titan mentioned by Rod Reiss had the ability to harden its skin and create the giant Walls, similar to that of the Wall Titans. The relationship between these Titans and that undoubtedly important Titan is currently unknown.
 * Titan Shifters - Titan Shifters are humans capable of transforming into Titans who have various unusual traits and skills that can be honed and directed with purpose. These Titans are usually stronger and/or faster than typical ones and retain their intelligence during a controlled transformation. They generally don't have any deformations that hinder their skills, and often resemble their human "pilot." A skilled Titan Shifter can move more quickly and regenerate parts of their body just by focusing, and even discard their body entirely and regain a new Titan form within minutes.

Nature and Creation
In the aftermath of the destruction of Ragako, the true nature of the Titans is discovered. The investigation performed by Hange Zoë 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 Hange speculates that the spinal cord operates 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 means of controlling the Titan body. The method used to slay Titans is revealed to sever the upper vertebrate of the human "pilot" (or what remains of them), killing them via decapitation.

It is stated by Ymir that the Beast Titan is the one responsible for transforming the villagers of Ragako, though the method employed remains a mystery. It is also unknown how Titan Shifters avoid being absorbed by their Titan forms, possibly a result of more advanced methods. It is theorized by Dot Pixis to be the only difference between Titan Shifters like Eren Yeager and regular Titans.

Hange theorizes that Titans can absorb abilities from their victims, allowing an ordinary Titan to regain human form by devouring a Titan Shifter. This was discovered when Ymir, then a regular Titan, devoured the young Titan Shifter Marceland regained human form. Hange theorizes that not only can Titan Shifting be transferred this way, but other unique traits such as Eren's Coordinate ability could be transferred in the same fashion. This is now confirmed as a fact by Rod Reiss and now it's known that memories will also be transferred to the one who ate the shifter, along with any powers they may possess. It is also possible that beliefs can be passed down if the person ate somebody of their bloodline, as the Reiss who possessed the Coordinate would be bound to the will of their ancestor, the First King, making them not want to exterminate the Titans, and never revealing to anybody the reasons why.

It is revealed that the injection that Grisha Yeager gave his son Eren was able to turn Eren into a normal Titan, allowing Eren to eat Grisha. Grisha, being a Titan Shifter, transformed Eren into a Titan Shifter and passed down the Coordinate and Frieda Reiss' memories at the cost of Grisha's life. The Reiss family has done this method for years, having one Reiss transform into a Titan via injection and eat the holder of the Coordinate, gaining Titan powers, memories of the lost world, and the Coordinate.

The serum in the injection is, most probably, the method through which most, if not all, Titans were created, though its way of administration can vary, as evidenced by the Beast Titan's inoculation of Ragako, implying that the agent can be spread through a multitude of mediums other than just a serum. Rod Reiss also gives insight into the vast multitude of physical structures and frames the Titans manifest, implying that it's actually possible for a serum to induce a certain kind of Titan, and in Rod's case, an immense Titan twice as big as the Colossal Titan. According to Hange, the serum is derived from human spinal fluid, but contains elements beyond the capability of their current science to analyse. The serum is also difficult to study because it begins to vaporize immediately upon contact with air.