Subjects of Ymir

The Subjects of Ymir (ユミルの民), also commonly referred to as Eldians (エルディア人) due to their history as the majority race of the Eldian Empire, are an ethnic group claiming descent from Ymir Fritz and the only race capable of being turned into Titans.

History
The Subjects of Ymir claim descent from Ymir Fritz herself, the first human to acquire the power of the Titans over 1,800 years ago. After Ymir's demise, her Subjects conquered the old kingdom of Marley using the power of the Titans and established an empire called Eldia, which over the centuries would come to rule the mainland continent of its origin.

The rule of the Eldian Empire lasted until the Great Titan War, which resulted in Marley's resurrection as a nation and Eldia's capital being moved to the island of Paradis. The land surrounding the new capital of Eldia would later be surrounded by three concentric Walls raised through the guidance of the Founding Titan. On the mainland, Marley gathered the Eldians, including the Subjects of Ymir, and placed them into internment zones such as within Liberio. They were spared eradication due to their ability to be turned into Titans for war purposes, as the Eldian Empire had done during its reign. On the island, the Subjects of Ymir had their memories of the world outside the Walls erased by the Founding Titan. The Eldians of other races were unaffected by the Founding Titan, so the royal family either granted them the privilege of living inside Wall Sheena in exchange for silence, such as the fake Fritz family, or eradicated them entirely, such as the Ackerman and Asian clans.

Special Characteristics
For reasons unknown, the Subjects of Ymir are the only humans capable of transforming into Titans. Nine of them (or less) will also inherit the Nine Titans. If the power of the Titans is not transferred before dying, the Titan will be sent to a random baby among the Subjects of Ymir, with distance and blood relation playing no factor. This led to the conclusion that all Subjects of Ymir are connected somehow, in ways invisible to the eyes.

One successor saw "paths". These "paths" send the organic material that makes up the body of a Titan and can even pass down abstract things such as memories or wills. According to Marley's Titan Chemistry Research Society, "paths" transcend physical space. They appear to transcend time as well, for Eren Kruger had "memories" of Mikasa Ackerman and Armin Arlert before the latter two were even born. "Paths" connect all Subjects of Ymir to a single coordinate, the Founding Titan.

Family relations appear to play a pivotal role in "paths". "Paths" that transfer the memories and wills of other Subjects of Ymir are significantly more effective when passing between family members. Some Titan abilities are even exclusive to specific families. The "will of the First King", the power of the Founding Titan, and the memories of the previous inheritors are all only fully activated when in possession of the royal family.

If a Subject of Ymir reproduces with an individual of another ethnicity, the offspring will also be a Subject of Ymir. An example of this is Reiner Braun, the bastard son of an Eldian mother (a Subject of Ymir) and a Marleyan father, a union which is against Marleyan law.

Subjects of Ymir can be identified with blood testing.

Ilse's Notebook
In the year 848, Ilse Langnar of the Survey Corps became the last surviving soldier of the left flank of the 2nd brigade during the 34th Expedition, lost in Titan territory within Wall Maria. With her vertical maneuvering equipment broken and horse missing, she chose to record her experiences as she made her way north toward Wall Rose. During her journey, she is caught by a Titan who forces her back against a tree but refrains from eating her. The Titan speaks to her, first believing her to be "a subject of Ymir" before mistaking her for "Lady Ymir" herself. Ilse attempts to communicate with the Titan while taking notes, asking what it is, where it comes from, and, after losing her composure, furiously asks why they eat humans. The Titan only responds with pained moaning and later starts ripping off its cheeks when Ilse screams at it. When she attempts to flee, the Titan catches her and crushes her head between its jaws, but does not consume her body. Her notebook and remains were discovered a year later by Hange Zoë and Levi Ackerman.

Trivia

 * It is commonly generalized that Eldians are Subjects of Ymir, possessing the ability to transform into Titans. However, this is not completely accurate as there are other races within the Eldian people. Even among the Eldians, Subjects of Ymir are avoided by other races to prevent their "filthy blood" from contaminating family lineages.
 * Annie Leonhart speculates that this is the reason for the non-Subject of Ymir Eldians within Paradis to stay in power: They do not mingle with Subjects of Ymir and do not trust outsiders who might be potential Subjects of Ymir. As such, nobody could get close to them to challenge their power.