Members of the cult served as the faithful subjects to their goddess "Ymir", who holds the blood of the king. By offering benevolence and care to Lady Ymir, they believe Ymir's royal blood will provide immortality for her subjects. The activities of the cult were apparently illegal in the society which the Subjects of Ymir resided in, punishable by exile and sentencing to life as a mindless Titan.
The Subjects of Ymir were highly benevolent and subservient to Lady Ymir, ensuring that she would live a happy and prosperous life as the figure of their worship. However, while the faith of the subjects was unwavering, the leader of the cult did not hold such zealous reverence for Ymir and was willing to endanger her life for the sake of his own.
Over sixty years ago, the leader of the Subjects of Ymir searched his hometown for a homeless girl which he would then give the name "Ymir". Upon bringing the girl to the cultists, the leader claimed that the child was Lady Ymir who possessed the blood of the king, able to grant them immortality. For several years, Lady Ymir was praised as a goddess and was offered the food and shelter she desired.
After this time, the Subjects of Ymir were discovered by a force of authorities who immediately apprehended the cultists. The leader claimed that Ymir had lied to them of her royalty, though Ymir stayed true to her name out of the hopes that the others would be spared. Unfortunately, her confession caused her to be taken with the cultists as they were led through the city streets until they came to a remote borderline. At this borderline, the cultists were given Titan injections and were sent to roam the world as mindless Titans, bringing about a brutal end for the cult. However, Ymir's Titan would after sixty years encounter the warriors on their way to breach Wall Maria. After eating Marcel, one of the warriors, Ymir miraculously returned to human form. She would later enter into the Walls and start a new life for herself.
In the year 849, Ilse Langnar of the Scout Regiment's 34th Exterior Scouting Mission encountered a Titan who had once been a Subject of Ymir sixty years ago. The Titan spoke to her, believing her to be "Lady Ymir" herself, 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. The findings in Ilse's notebook convince Hange and Commander Erwin Smith that attempted Titan capture should be resumed as a Scout Regiment practice.