Power of the Titans

Titan Shifters are humans in possession of the "Titan's power" (巨人の力), which allows them to transform into Titans. Unlike regular Titans, they are able to control their regular form and retain their human intelligence. It is often noted that Shifters are more powerful than regular Titans.

Description
To transform, they must suffer an injury that draws blood (often self-inflicted, as shown when Eren Yeager bit his hand to protect Mikasa Ackerman and Armin Arlert, and when Annie Leonhart stabbed her finger with a blade from the inside of her ring) while having a solid goal or belief in mind at the moment of injury. However, being injured by another person is also sufficient, such as when Mikasa seriously wounded Reiner and Bertolt. After transformation, their human bodies appear to be in the Titan's nape of the neck, where they control their Titan bodies. Their human bodies are also merged with the Titan's flesh inside, rendering them immobile. It is also visible that their eyes are connected within the body of the Titan, presumably for optic vision outside the body.

If a Shifter's Titan form is pushed to its limit, its human form will emerge from the nape of its neck in a semi-conscious state. This is demonstrated separately by Eren, Ymir, and Annie when their Titan forms suffer defeat. But if a Shifter's Titan form is not exhausted, then its human form can partially emerge from the Titan form in full consciousness. Annie, Ymir, and even Eren have shown this ability on three separate occasions.

The Titan Shifters also gain the Titan's ability to regenerate, capable of vastly improved healing abilities and regenerating lost limbs even in their human forms. However, regeneration from severe injuries such as lost limbs can strain their powers and prevent them from transforming for a period of time. Both Eren and Ymir were unable to transform while recovering from their injuries, though Ymir is shown to have been able to transform within minutes of recovering.

It is also shown that they are not safe from regular Titans, and will be attacked even while transformed. Both Eren and Ymir were nearly devoured by hordes of Titans while transformed, and Reiner states that traveling while in Titan-occupied territory is dangerous to any of them. During the battle of Trost, it was shown that Titans seem to prefer attacking transformed Shifters. The human soldiers had great difficulty drawing away the Titans, as they were somehow drawn to Eren's Titan form.

A notable trait of the three major antagonistic Shifters is their lack of skin, and special abilities unique only to their Titans. It is unknown how this relates to Eren and Ymir, who do not share their place of origin or purpose. Ymir displays the ability to partially emerge from the nape of her Titan form, allowing her to interact with others while still transformed. It is unknown whether this is unique to her or a feat that any experienced Shifter can manage.

Revelations concerning Ymir's past also suggest several important possibilities concerning the nature of Titans and Titan Shifters, as well as the existence of other Shifters. It is revealed that they do not age while in their Titan form, as Ymir notes she spent 60 years in her Titan form but has the appearance of a teenager. Although Ymir lacks complete and clear memory of this period, she recalls enough to describe it as an "endless nightmare." While trapped in her Titan form, she seems to have been a regular Titan that mindlessly devoured humans. Reiner and Bertolt do not treat this as an unusual case, and Bertolt notes that it was the same for them. Which leads to the conclusion that the first time of using their power they lack self-determination and have only a decided goal, in Eren's case it was to kill Titans.

Experiments performed on Eren suggest that Shifters are resistant, but not immune to the normal absorption that creates mindless Titans. The strength and quality of his Titan form degraded with each subsequent transformation, becoming smaller and less controlled each time. On the third transformation, his lower body remained free while his upper body began to merge with his Titan form. It became necessary to cut him loose, removing most of the tissue from the front of his skull in the process. As such, it seems that there is a limitation on how long a Shifter can remain transformed before risking absorption.

Their powers can be transferred to a new host, should they be devoured. This allows an ordinary Titan to become a new Titan Shifter, should they succeed in devouring one—as was the case when Ymir devoured Marcel and regained human form. Hange Zoë theorizes other abilities, such as the Coordinate possessed by Eren, could be transferred to a new host in the same fashion.

Grisha Yeager, a Shifter himself, was capable of using a syringe to turn a young Eren into a Titan which in turn devoured most of his body. Similarly, Frieda Reiss became a Titan via syringe in order to consume her uncle to become a Shifter and to obtain the Coordinate ability. Rod Reiss revealed that Titan Shifters who have been given their abilities by having to devour someone have a chosen Titan Shifter form for them to take.

The Homeland
Little is known about the place where the majority of known Titan Shifters originated. It is said to be located somewhere outside Wall Maria, and is alternately referred to as the "village" or "homeland" of Reiner Braun, Bertolt Hoover, Annie Leonhart, and Marcel. It is one of the greatest mysteries of the series, with little revealed about it. Flashbacks to Annie's childhood show the clothing styles and housing are similar to that of the people living within Wall Maria. The only certainty is that there are in fact humans surviving outside of the walls, whether they are also Titan Shifters or not is unknown.

The people from this land refer to themselves as being "warriors", a distinction also made when Ymir refers to them. While it is unknown whether all those living there are Titan Shifters, all three surviving characters from that location possess this ability. Ymir states that they possess the "Power of Titans", and she obtained her powers by stealing from them. As a result, she is a fugitive and greatly fears being killed by them should they catch her. She believes that Historia Reiss may be valuable enough to them to earn a pardon, and would not be harmed if surrendered to them.

In 845, they launched an attack against humanity and sent at least three agents into the Walls in search of the Beast Titan and something they refer to as the "Coordinate". However, Ymir believes that they possess influence even within humanity's lands, claiming that no one is safe from them regardless of where they might hide. Their agents are similarly desperate to complete their mission, going to suicidal extremes in their quest to obtain Eren Yeager even though they have expressed doubts that he is indeed the Coordinate.

When Eren manifests the power of the Coordinate, he gains the ability to command a horde of mindless Titans. Erwin Smith suggests it also has the potential to control normal humans, and had some role in not only the creation of the Walls, but also the memory loss suffered by humanity when they settled within them. While its exact nature remains a mystery, the Coordinate sought by the Titan Shifters is spoken of as a possession, and mentioned to have been taken from them.