Subjects of Ymir

The Subjects of Ymir (ユミルの民) is an ethnic group claiming descent from Ymir Fritz and the only race capable of being turned into Titans. After Ymir's demise, her Subjects conquered the old Marley using the power of the Titans and established an empire called Eldia. They comprise the majority of the Eldian people.

History
Subjects of Ymir originated from Ymir Fritz herself, the first human to acquire the power of the Titans.

Titans and the Paths
For reasons unknown, the Subjects of Ymir are the only humans capable of transforming into Titans. In ancient times, Subjects of Ymir that were turned into Titans were used as unstoppable weapons, though after the Great Titan War all Titans seem to exist only on the island of Paradis, excluding the seven of the Nine Titans taken by Marley in the war, and the Attack Titan, which does not fall in Marley's hands.

The power of the Titans stems from an abstract phenomena referred to only as "paths invisible to the eyes". Through these unseen paths transcending both time and space, the organic material which makes up the body of a Titan is sent to its user. Since Ymir Fritz was the first of all Titans, it is likely that the paths from which the organic matter originates may be the "source of all living matter" which she found 2,000 years ago.

The paths also act as a fail-safe for the "Curse of Ymir", which prevents holders of Titan power from keeping their power for longer than the 13 years Ymir Fritz held her Titan power. If the holder of an intelligent Titan dies before their Titan can be inherited, the power of that Titan follows the unseen paths and is reborn into a baby Subject of Ymir, with distance and blood relation playing no factor into which child is born with the power. Furthermore, the paths are capable of sending memories or willpower from one individual to another across time and space, even backwards in time. In this region beyond physical space, all paths linked to a single coordinate known as the Founding Titan, which has the power over the memories and willpower of Subjects of Ymir.

Blood and family
Subjects of Ymir can be identified with blood testing. If a Subject of Ymir reproduces, the child will also be a Subject of Ymir.

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

 * Subjects of Ymir and Eldians are often referred to each other; however, this is only a generalization 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 trees.