Subjects of Ymir

"Get a little Titan spinal fluid in your blood and you become giant monsters. And you think you're just as human as us? There is no living thing in the world like you Eldians, you "Subjects of Ymir." You're monsters in human skin. And it's a terrifying nightmare that anyone ever let you reproduce."

- Sergeant Major Gross to Grisha Yeager

The Subjects of Ymir (ユミルの民) are an ethnic group descended from Ymir Fritz and the only race capable of becoming Titans. They built Eldia, and are commonly referred to as Eldians (エルディア人) alongside other minority ethnicities and races. Currently, they populate the Walls of Paradis and the internment zones found in Marley and other nations of the world.

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 Nine 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, 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 in the case of the Ackerman family and Asian clan.

Titans and Paths
The Subjects of Ymir are the only humans capable of transforming into Titans. Some of them will also inherit the Nine Titans. If one of the Nine 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 Biology 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 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 power of the Founding Titan and Karl Fritz's vow of pacifism are only fully active when in possession of the royal family.

Ethnicity
If a Subject of Ymir reproduces with an individual of another ethnicity, the offspring will be a Subject of Ymir. An example of this is Reiner Braun, the illegitimate son of an Eldian mother and a Marleyan father. When Eldia was in power, the Eldians forced other races to have their children so that there will be more Subjects of Ymir.

Subjects of Ymir can be identified through 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.

Return to Shiganshina arc
Upon uncovering the secrets of Grisha Yeager's basement following the Battle of Shiganshina District, the surviving members of the Survey Corps learn of their ethnic identity as Subjects of Ymir, a race despised by all peoples of the world. This news, along with the many remarkable truths found in the basement, become a much-discussed topic among people of varying opinions within the Walls.

Marley arc
Following the Marley Mid-East War, the Warriors of Marley begin to grow concerned over the fate of the Subjects of Ymir, particularly the world's growing desire to see their race exterminated in order to end the Titan threat. Capturing the Founding Titan remains in high priority, though Pieck acknowledges that this alone will not save the reputation of the Subjects of Ymir. In an effort to shed a positive light on the Eldian peoples and to do away with the threat posed by the people of Paradis, Zeke Yeager collaborates with the Tybur family, who agree to make a public announcement in favor of the Eldians during a festival in Liberio. At this time, they will convince the nations of the world that Paradis will be conquered within a year. Soon afterwards, the Tybur family comes to Liberio. As Commander Theo Magath and Lord Willy Tybur discuss the affairs of their nation, Lord Tybur expresses the regret he and his ancestors have felt for the fate the Subjects of Ymir have been burdened with in the past century. Desiring to set things right for his nation, Tybur offers to work with Magath towards the betterment of Marley.

One month later, the time of the festival approaches. On the eve of the festival, Willy acknowledges the world's shared hatred for the Eldian people, announcing that by the end of the festival he will announce his plan to solve the crisis posed by the Subjects of Ymir and the Titans.

Trivia

 * Although the Subjects of Ymir are commonly refered to as "Eldians", not all Eldians are Subjects of Ymir. Other races who are subservient to the Eldian King also make up the Eldian people. They are considered minorities.
 * Even among the Eldians, Subjects of Ymir are avoided to prevent their "filthy blood" from contaminating the other races.
 * Annie Leonhart speculates that this is how Eldians within Paradis who are not Subjects of Ymir stay in power: they do not mingle with Subjects of Ymir and do not trust outsiders who might be of the bloodline. As such, nobody could get close to them to challenge their power.