The Subjects of Ymir, also referred to as the Eldians, are an ethnic group that descended from Ymir Fritz and the only race that was capable of transforming into Titans. Beginning with Ymir and her three daughters, the bloodline eventually expanded across the Eldian people, to the point of majority. In 854, they populated the Walls of Paradis Island and the internment zones found in Marley and other nations of the world.
The Subjects of Ymir believe that they are the "chosen children of God"; however, the rest of the world views them as an "evil" race of devils, the "spawn of the Devil".
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 daughters Maria, Rose, and Sina Fritz devoured her corpse and gained her ability to turn into a Titan. As per their father's commands, Ymir's daughters reproduced to expand the bloodline and continue the race of humans who can turn into Titans.
Story: " The Titan spoke to her, believing her to be "Lady Ymir," and Ilse attempted to question it. After Ilse panics when the Titan gives no answers, the Titan kills Ilse bitterly before storing her corpse in a nearby tree. One year later, Ilse's notebook recounting her time in the scouting mission is found by Section Commander Hange Zoë and Captain Levi on the 49th Exterior Scouting Mission.
Trivia: In modern times, the Subjects of Ymir are often simply called "Eldians". While this is generally true as most Eldians are Subjects of Ymir, the two terms are not entirely synonymous. According to Reiner Braun, Bertholdt Hoover, and Annie Leonhart, the false king and the nobles who ruled the interior of the Walls are Eldians of another race.
Source: Subjects of Ymir (Anime), available under CC BY-SA 3.0.