Frieda Reiss was Rod Reiss's eldest daughter and Historia's oldest half-sister. She was the true queen of the Walls from 842 to 845 and the last of the Founding Titans before it was merged with the Attack Titan. She is known to appear in Eren and Historia's obtained memories.
When Eren dreams of her, he first mistakes Frieda for Historia, highlighting the sisters' physical similarity. Frieda was tall and had long, smooth, dark hair; stopping well past her shoulders but she later cut it shoulder length before eating her uncle Uri and prior to her death her hair had grown back about half its original length. Frieda had a rounded face and was usually seen in simple dresses and skirts despite being a noble. After receiving the Founding Titan, Frieda's eyes would differentiate between light and a dark vacant, a sign she had inherited the memories of the world as well as the will of the First King.
Frieda tended to be a good and caring person. She liked to walk and look after the peasants. She loved Historia to the point that she erased her memories in order to protect her. She possibly volunteered to become the next inheritor of the Founding Titan. She was brave, as she believed she would not give in to her ancestor's ghosts. She cried during the succession ceremony, as it meant she had to eat her uncle.
Appearance: When Eren dreams of her, he first mistakes Frieda for Historia, highlighting the sisters' physical similarity. Frieda was tall and had long, smooth, dark hair; stopping well past her shoulders but she later cut it shoulder length before eating her uncle Uri and prior to her death her hair had grown back about half its original length.
Personality: Frieda tended to be a good and caring person. She liked to walk and look after the peasants.
History: Frieda taught Historia Reiss how to read and told her to grow up to be "ladylike" and a kind person. She appeared to be moved when Historia claims that she wants to be like her, then proceeded to manipulate Historia's memory, telling her to forget about her until they see each other again.
Frieda Reiss in the story: Frieda is present with the Reiss family the night Grisha Yeager raids the Reiss chapel. She listens as Grisha pleads with the Reiss family to protect them against the Titan invasion following the Fall of Wall Maria. For a moment, Frieda appears willing to agree to Grisha's request, but then she seems to be forced to change her mind due to the First King's Will.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.