Eldia

, also known as the "subjects of Ymir", are an unidentified group from outside the Walls.

History
Over 60 years before the fall of the Shiganshina District, Ymir was cast out by her people, who "hated her just for existing". She would wander outside the Walls for 60 years until the fall of Wall Maria in the year 845. The Homeland, a society outside of the walls, sent Reiner Braun, Bertolt Hoover, Annie Leonhart, and Marcel to breach the southern gate of Shiganshina where, as a Titan, she would steal the power of Titan shifting from Marcel after catching and consuming him. After this, Ymir regained her conscious mind and became a part of the society within the walls.

Side Story: Ilse's Notebook
In the year 848, Ilse Langnar of the Survey Corps becomes 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 3D maneuver gear broken and horse missing, she chooses to record her experiences as she makes 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 who it is, where it comes from, and, after losing her composure, furiously asks why it eats humans. The Titan only response with pained moaning and later starts ripping opp its cheeks when Ilse screams at it. When she attempts to flee, the Titan catches her and crushes her head between, but does not consume her body. Her notebook and remains were discovered a year later by Hange Zoë and Levi Ackerman.